fix(codegen): Support extern crate serde not in toplevel module

[dtolnay]

Addresses #159.

The generated code looks like:

#[allow(non_upper_case_globals, unused_attributes, unused_qualifications)]
const _IMPL_DESERIALIZE_FOR_MyStruct: () = {
    extern crate serde as _serde;
    #[automatically_derived]
    impl _serde::de::Deserialize for MyStruct {
        // ...
    }
};

[oli-obk]

why was this removed? Did it cause an error?

[dtolnay]

No error, just to demonstrate that things work even without "extern crate serde" at the top level. I kept the equivalent line in bench.rs so the build still uses both approaches.

[oli-obk]

Does this break any existing code? You changed quite a few tests' imports.

[dtolnay]

This does not break any existing code.

[dtolnay]

This makes extern crate serde optional unless you are using it explicitly in your own code, so I have removed it from the example in the readme.

[erickt]

@dtolnay: Once again, great work here. I'm working on merging this patch in by hand.

[dtolnay]

@erickt I fixed the merge conflict myself.

[dtolnay]

Ping @erickt - looks like you said this was ready to merge. Once you merge it, I will resolve the merge conflict in #293 (comment).

[erickt]

Thanks @dtolnay! I've been swamped with work recently, and unfortunately that's resulted in lots of my projects suffering. Thanks for keeping up with this.