fix(macro): use FQDNs when calling contract methods

[mitinarseny]

Hey Team!

I've noticed a small inconsistency while working on #[access_control] from near-plugins, which uses #[near-sdk::near] macro internally.

The inconsistency itself can be described as follows:

The generated code by #[near_bindgen] assumes that all necessary traits were imported into current scope.

To simplify the issue, consider the following example:

use near_sdk::near;

pub mod other {
    pub trait SomeTrait {
        fn method(&self);
    }
}

#[near(contract_state)]
struct Contract {
    // ..
}

#[near]
impl other::SomeTrait for Contract {
    fn method(&self) {
        // ...
    }
}

When you compile it with cargo build --target wasm32-unknown-unknown, you get the following error:

error[E0599]: no method named `method` found for struct `Contract` in the current scope
  --> controller/src/lib.rs:17:8
   |
5  |         fn method(&self);
   |            ------ the method is available for `Contract` here
...
11 | struct Contract {
   | --------------- method `method` not found for this struct
...
17 |     fn method(&self) {
   |        ^^^^^^ method not found in `Contract`
   |
   = help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope; perhaps add a `use` for it:
   |
1  + use crate::other::SomeTrait;
   |

For more information about this error, try `rustc --explain E0599`.
error: could not compile `defuse-controller-contract` (lib) due to previous error

This happens due to method is not implemented on Contract itself but is a part of impl other::SomeTrait for Contract, so it's not visible in the current scope.
Of course, It can be fixed by adding use other::SomeTrait at top of your module, but it's better to avoid it, so it can just work.

The current version of near-sdk 5.1.0 generates following code:

#[cfg(target_arch = "wasm32")]
#[no_mangle]
pub extern "C" fn method() {
    ::near_sdk::env::setup_panic_hook();
    let contract: Contract = ::near_sdk::env::state_read().unwrap_or_default();
    contract.method();
}

This PR introduces a fix for that by using fully qualified syntax for calling methods that are part of trait implementation.

Here is the code that this PR generates:

#[cfg(target_arch = "wasm32")]
#[no_mangle]
pub extern "C" fn method() {
    ::near_sdk::env::setup_panic_hook();
    let contract: Contract = ::near_sdk::env::state_read().unwrap_or_default();
    <Contract as other::SomeTrait>::method(&contract);
}

[frol]

@mitinarseny Great job! Thank you for your contribution! It seems to resolve #1084