Add support for cfg_attr for struct fields

[fl0rek]

closes #2703

Overview

• During #[wasm_bindgen] attribute macro expansion on struct, instead of generating ast for Program, it puts a #[derive(BindgenedStruct)] on it and preserves all the wasm_bindgen atributes as-is.
• When #[derive(BindgenedStruct)] is expanded, cfg_attr are already evaluated, so macro can just processes the leftover attributes and generate the glue code required, as it's done now. Attributes are then automatically removed by the compiler, since macro is defined as #[proc_macro_derive(BindgenedStruct, attributes(wasm_bindgen))].

[daxpedda]

Thank you, this is a great improvement!

[fl0rek]

@daxpedda thanks for the review! Pushed the update after the prolonged xmas break

[daxpedda]

Thank you!

[daxpedda]

Ah, apparently this requires a rebase! Otherwise LGTM!