Fix conditional #[wasm_bindgen] in impls

Reported in #1191 the fix requires us to get a bit creative I think. The
general gist is that a block like this:

    #[wasm_bindgen]
    impl Foo {
        pub fn foo() {}
    }

was previously expanded all in one go. Now, however, it's expanded into:

    impl Foo {
        #[__wasm_bindgen_class_marker(Foo = "Foo")]
        pub fn foo() {}
    }

    // goop generated by orginal #[wasm_bindgen]

This method of expansion takes advantage of rustc's recursive expansion
feature. It also allows us to expand `impl` blocks and allow inner items
to not be fully expanded yet, such as still having `#[cfg]` attributes
(like in the original bug report).

We use theinternal `__wasm_bindgen_class_marker` to indicate that we're
parsing an `ImplItemMethod` unconditionally, and then generation
proceeds as usual. The only final catch is that when we're expanding in
an `impl` block we have to generate tokens for the `Program`
(wasm-bindgen injected goop like the custom section) inside the body
of the function itself instead of next to it. Otherwise we'd get syntax
errors inside of impl blocks!

Closes #1191

crates/macro/src/lib.rs

@@ -19,3 +19,16 @@ pub fn wasm_bindgen(attr: TokenStream, input: TokenStream) -> TokenStream {
         Err(diagnostic) => (quote! { #diagnostic }).into(),
     }
 }
+
+#[proc_macro_attribute]
+pub fn __wasm_bindgen_class_marker(attr: TokenStream, input: TokenStream) -> TokenStream {
+    match macro_support::expand_class_marker(attr.into(), input.into()) {
+        Ok(tokens) => {
+            if cfg!(feature = "xxx_debug_only_print_generated_code") {
+                println!("{}", tokens);
+            }
+            tokens.into()
+        }
+        Err(diagnostic) => (quote! { #diagnostic }).into(),
+    }
+}