Allow renaming exported functions into JS (#525)

Support the `js_name` attribute on exports as well as imports to allow exporting
types as camelCase instead of snake_case, for example.

Closes #221

crates/backend/src/codegen.rs

@@ -297,7 +297,7 @@ impl ToTokens for ast::Export {
             0
         };
 
-        let name = &self.function.name;
+        let name = &self.rust_name;
         let receiver = match self.method_self {
             Some(ast::MethodSelf::ByValue) => {
                 let class = self.class.as_ref().unwrap();