Start optimizing code size:

* Use a bundled custom `WasmRefCell` instead of the one in the standard library.
  This one primarily doesn't panic via libstd which means that its code
  footprint is much smaller.
* Add a `throw` function to `wasm_bindgen`-the-crate which can be used to throw
  an exception in JS from Rust. This is useful as a cheap way to throw an
  exception code-wise (little code bloat) and it's also a great way of reporting
  error messages to JS!
* Cut down on the code size of `__wbindgen_malloc` by aborting on huge requests
  earlier.
* Use a custom `assert_not_null` function which delegates to `throw` to test for
  incoming null pointers

crates/wasm-bindgen-cli-support/src/ts.rs

@@ -389,6 +389,13 @@ impl Js {
             imports.push_str("__wasm_bindgen_object_drop_ref: dropRef,\n");
         }
 
+        if self.wasm_import_needed("__wbindgen_throw", m) {
+            self.expose_get_string_from_wasm();
+            imports.push_str("__wbindgen_throw: function(ptr: number, len: number) {
+                throw new Error(getStringFromWasm(ptr, len));
+            },\n");
+        }
+
         let mut writes = String::new();
         if self.exposed_globals.contains(&"memory") {
             writes.push_str("memory = exports.memory;\n");