Import wasm_bindgen in macros

[coolreader18]

This would fix (or make unnecessary) brendanzab/gl-rs#528, which was an attempt to fix webgl_stdweb compiling under wasm-pack/wasm-bindgen. It was giving me these kind of errors:

❯ cargo c --target wasm32-unknown-unknown
    Checking webgl_stdweb v0.3.0 (/home/coolreader18/gl-rs/webgl_stdweb)
error[E0433]: failed to resolve: use of undeclared type or module `wasm_bindgen`
    --> /home/coolreader18/gl-rs/target/wasm32-unknown-unknown/debug/build/webgl_stdweb-fd022828f1f02c5e/out/bindings.rs:5789:9
     |
5789 | /         js!(
5790 | |             return true;
5791 | |         ).try_into().unwrap()
     | |_________^ use of undeclared type or module `wasm_bindgen`
     |
     = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0433]: failed to resolve: use of undeclared type or module `wasm_bindgen`
    --> /home/coolreader18/gl-rs/target/wasm32-unknown-unknown/debug/build/webgl_stdweb-fd022828f1f02c5e/out/bindings.rs:5769:24
     |
5769 | #[derive(Debug, Clone, ReferenceType)]
     |                        ^^^^^^^^^^^^^ use of undeclared type or module `wasm_bindgen`
     |
     = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0433]: failed to resolve: use of undeclared type or module `wasm_bindgen`
    --> /home/coolreader18/gl-rs/target/wasm32-unknown-unknown/debug/build/webgl_stdweb-fd022828f1f02c5e/out/bindings.rs:5760:24
     |
5760 | #[derive(Debug, Clone, ReferenceType)]
     |                        ^^^^^^^^^^^^^ use of undeclared type or module `wasm_bindgen`
     |
     = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)
... 1600 total errors 

I was able to patch it by just adding wasm-bindgen as a dep to webgl_stdweb, but I realized I was also able to fix it directly in stdweb by importing wasm_bindgen inside the js!()/derive(ReferenceType) macros. I think it was failing because wasm-bindgen's macros expect to be able to access wasm_bindgen, but because crates invoking the stdweb macros that invoke the wasm-bindgen macros don't necessarily have wasm_bindgen in scope, it might fail to compile.

[coolreader18]

Oh... I guess this is related to/duplicate of #390

[coolreader18]

@koute could you review this?