Add a raw_module attribute to #[wasm_bindgen]

This allows subverting the checks and resolution performed by the
`module` attribute added as part of [RFC 6] and has been discussed in #1343.

Closes #1343

[RFC 6]: https://github.com/rustwasm/rfcs/pull/6

crates/backend/src/ast.rs

@@ -79,6 +79,7 @@ pub struct Import {
 pub enum ImportModule {
     None,
     Named(String, Span),
+    RawNamed(String, Span),
     Inline(usize, Span),
 }
 
@@ -96,6 +97,10 @@ impl Hash for ImportModule {
                 2u8.hash(h);
                 idx.hash(h);
             }
+            ImportModule::RawNamed(name, _) => {
+                3u8.hash(h);
+                name.hash(h);
+            }
         }
     }
 }