Fix clippy for generated code

[tarikeshaq]

Right now running cargo clippy in a project that uses uniffi results in a few clippy errors, we could either try to fix them, or just disable clippy warnings in any spot that we can't cleanly make it happy. (We could also just say that consumers can deal with it as well if it's not worth our time)

Here is a long and ugly log from running clippy with the nimbus SDK:

/Users/teshaq/code/nimbus-sdk/experiments/target/debug/build/experiments-e671f8ff694721e7/out/nimbus.uniffi.rs:343:3
    |
343 | ) -> () {
    |   ^^^^^ help: remove the `-> ()`
    |
    = note: `#[warn(clippy::unused_unit)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit

warning: returning the result of a `let` binding from a block
   --> /Users/teshaq/code/nimbus-sdk/experiments/target/debug/build/experiments-e671f8ff694721e7/out/nimbus.uniffi.rs:357:9
    |
352 | /         let _retval = Experiments::opt_in_with_branch(
353 | |             obj,
354 | |             <String as uniffi::ViaFfi>::try_lift(experiment_slug).unwrap(),
355 | |             <String as uniffi::ViaFfi>::try_lift(branch).unwrap(),
356 | |         );
    | |__________- unnecessary `let` binding
357 |           _retval
    |           ^^^^^^^
    |
    = note: `#[warn(clippy::let_and_return)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_and_return
help: return the expression directly
    |
352 |         
353 |         Experiments::opt_in_with_branch(
354 |             obj,
355 |             <String as uniffi::ViaFfi>::try_lift(experiment_slug).unwrap(),
356 |             <String as uniffi::ViaFfi>::try_lift(branch).unwrap(),
357 |         )
    |

warning: unneeded unit return type
   --> /Users/teshaq/code/nimbus-sdk/experiments/target/debug/build/experiments-e671f8ff694721e7/out/nimbus.uniffi.rs:365:3
    |
365 | ) -> () {
    |   ^^^^^ help: remove the `-> ()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit

warning: returning the result of a `let` binding from a block
   --> /Users/teshaq/code/nimbus-sdk/experiments/target/debug/build/experiments-e671f8ff694721e7/out/nimbus.uniffi.rs:378:9
    |
374 | /         let _retval = Experiments::opt_out(
375 | |             obj,
376 | |             <String as uniffi::ViaFfi>::try_lift(experiment_slug).unwrap(),
377 | |         );
    | |__________- unnecessary `let` binding
378 |           _retval
    |           ^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_and_return
help: return the expression directly
    |
374 |         
375 |         Experiments::opt_out(
376 |             obj,
377 |             <String as uniffi::ViaFfi>::try_lift(experiment_slug).unwrap(),
378 |         )
    |

warning: unneeded unit return type
   --> /Users/teshaq/code/nimbus-sdk/experiments/target/debug/build/experiments-e671f8ff694721e7/out/nimbus.uniffi.rs:383:63
    |
383 | pub extern "C" fn nimbus_Experiments_opt_out_all(handle: u64) -> () {
    |                                                               ^^^^^ help: remove the `-> ()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit

warning: returning the result of a `let` binding from a block
   --> /Users/teshaq/code/nimbus-sdk/experiments/target/debug/build/experiments-e671f8ff694721e7/out/nimbus.uniffi.rs:393:9
    |
392 |         let _retval = Experiments::opt_out_all(obj);
    |         -------------------------------------------- unnecessary `let` binding
393 |         _retval
    |         ^^^^^^^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_and_return
help: return the expression directly
    |
392 |         
393 |         Experiments::opt_out_all(obj)
    |

warning: unneeded unit return type
   --> /Users/teshaq/code/nimbus-sdk/experiments/target/debug/build/experiments-e671f8ff694721e7/out/nimbus.uniffi.rs:401:3
    |
401 | ) -> () {
    |   ^^^^^ help: remove the `-> ()`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unused_unit

/ pub unsafe extern "C" fn ffi_nimbus_string_alloc_from(
28 | |     cstr: *const std::os::raw::c_char,
29 | |     err: &mut uniffi::deps::ffi_support::ExternError,
30 | | ) -> *mut std::os::raw::c_char {
...  |
44 | |     })
45 | | }
   | |_^
   |
   = note: `#[warn(clippy::missing_safety_doc)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_safety_doc

warning: unsafe function's docs miss `# Safety` section
  --> /Users/teshaq/code/nimbus-sdk/experiments/target/debug/build/experiments-e671f8ff694721e7/out/nimbus.uniffi.rs:52:1
   |
52 | / pub unsafe extern "C" fn ffi_nimbus_string_free(cstr: *mut std::os::raw::c_char) {
53 | |     // We deliberately don't check the `Result` here, so that callers don't need to pass an out `err`.
54 | |     // There was nothing for us to free in the failure case anyway, so no point in propagating the error.
55 | |     let s = <String as uniffi::ViaFfi>::try_lift(cstr);
56 | |     drop(s)
57 | | }
   | |_^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#missing_safety_doc

warning: passing a unit value to a function
   --> /Users/teshaq/code/nimbus-sdk/experiments/target/debug/build/experiments-e671f8ff694721e7/out/nimbus.uniffi.rs:408:12
    |
408 |         Ok(_retval)
    |            ^^^^^^^
    |
    = note: `#[warn(clippy::unit_arg)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unit_arg
help: if you intended to pass a unit value, use a unit literal instead
    |
408 |         Ok(())

┆Issue is synchronized with this Jira Task

[eoger]

Seems like these errors come from these 2 macros, but it's kinda hard to fix. I would suggest simply disabling clippy as we don't really expect the consumer of these bindings to read the source code directly.