Introduce the wasm-bindgen-webidl frontend

This is still a work in progress. Parse WebIDL source text and convert it into
wasm-bindgen AST, so that we can automatically emit bindings for the types and
functions described in the WebIDL.

crates/webidl/tests/all/lib.rs

@@ -0,0 +1,21 @@
+extern crate proc_macro2;
+extern crate syn;
+extern crate wasm_bindgen_backend as backend;
+extern crate wasm_bindgen_webidl as wb_webidl;
+
+pub fn assert_parse(webidl: &str, expected: backend::ast::Program) {
+    let actual = wb_webidl::parse(webidl).expect("should parse the webidl source OK");
+    assert_eq!(expected, actual);
+}
+
+macro_rules! assert_parse {
+    ($test_name:ident, $webidl_source:expr, $expected_ast:expr) => {
+        #[test]
+        fn $test_name() {
+            $crate::assert_parse($webidl_source, $expected_ast);
+        }
+    };
+}
+
+mod event;
+mod simple;