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

tests/wasm/classes.js

@@ -143,3 +143,8 @@ exports.js_renamed_export = () => {
     x.foo();
     x.bar(x);
 };
+
+exports.js_conditional_bindings = () => {
+    const x = new wasm.ConditionalBindings();
+    x.free();
+};

tests/wasm/classes.rs

@@ -22,6 +22,7 @@ extern "C" {
     fn js_js_rename();
     fn js_access_fields();
     fn js_renamed_export();
+    fn js_conditional_bindings();
 }
 
 #[wasm_bindgen_test]
@@ -402,3 +403,19 @@ impl RenamedExport {
 fn renamed_export() {
     js_renamed_export();
 }
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen)]
+pub struct ConditionalBindings {
+}
+
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen)]
+impl ConditionalBindings {
+    #[cfg_attr(target_arch = "wasm32", wasm_bindgen(constructor))]
+    pub fn new() -> ConditionalBindings {
+        ConditionalBindings {}
+    }
+}
+#[wasm_bindgen_test]
+fn conditional_bindings() {
+    js_conditional_bindings();
+}