Remove the need for a ConstructorToken

This commit removes the need for an injected `ConstructorToken` type and
also cleans up the story we have for generating constructors a bit.
After this commit a `constructor()` is omitted entirely if we're in
non-debug mode and there's no actual listed constructor. Additionally we
don't deal with splat arguments and rerouting constructors, Nick was
kind enough to enlighten me about `Object.create` which is creating an
instance without running the constructor!

Instances of an exported type are now created through one of two
methods:

* If `#[wasm_bindgen(constructor)]` is present, then a `constructor` is
  generated with the appropriate signature. If a constructor is not
  present and we're in debug mode, a throwing constructor is generated.
  If we're in release mode and there's no constructor, no constructor is
  generated.

* Otherwise if a binding returns an instance of a type (or otherwise
  needs to manfuacture an instance, then it will cause an internal
  `__wrap` function to be generated. This function will use
  `Object.create` to create an instance without running the constructor.

This should ideally clean up our generated JS for classes quite a bit,
making it much more lean-and-mean!

crates/cli/src/bin/wasm-bindgen-test-runner/server.rs

@@ -29,7 +29,7 @@ pub fn spawn(
             // native ESM support for wasm modules.
             await wasm.booted;
 
-            const cx = Context.new();
+            const cx = new Context();
             window.console_log_redirect = __wbgtest_console_log;
             window.console_error_redirect = __wbgtest_console_error;