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/backend/src/ast.rs

@@ -34,11 +34,9 @@ pub struct Export {
     pub class: Option<Ident>,
     /// The type of `self` (either `self`, `&self`, or `&mut self`)
     pub method_self: Option<MethodSelf>,
-    /// The name of the constructor function (e.g. new).
-    ///
-    /// This allows javascript to expose an `Object` interface, where calling
-    /// the constructor maps correctly to rust.
-    pub constructor: Option<String>,
+    /// Whether or not this export is flagged as a constructor, returning an
+    /// instance of the `impl` type
+    pub is_constructor: bool,
     /// The rust function
     pub function: Function,
     /// Comments extracted from the rust source.
@@ -317,7 +315,7 @@ impl Export {
             class: self.class.as_ref().map(|s| s.to_string()),
             method,
             consumed,
-            constructor: self.constructor.clone(),
+            is_constructor: self.is_constructor,
             function: self.function.shared(),
             comments: self.comments.clone(),
         }