Overhaul how type information gets to the CLI

This commit is a complete overhaul of how the `#[wasm_bindgen]` macro
communicates type information to the CLI tool, and it's done in a somewhat...
unconventional fashion.

Today we've got a problem where the generated JS needs to understand the types
of each function exported or imported. This understanding is what enables it to
generate the appropriate JS wrappers and such. We want to, however, be quite
flexible and extensible in types that are supported across the boundary, which
means that internally we rely on the trait system to resolve what's what.

Communicating the type information historically was done by creating a four byte
"descriptor" and using associated type projections to communicate that to the
CLI tool. Unfortunately four bytes isn't a lot of space to cram information like
arguments to a generic function, tuple types, etc. In general this just wasn't
flexible enough and the way custom references were treated was also already a
bit of a hack.

This commit takes a radical step of creating a **descriptor function** for each
function imported/exported. The really crazy part is that the `wasm-bindgen` CLI
tool now embeds a wasm interpreter and executes these functions when the CLI
tool is invoked. By allowing arbitrary functions to get executed it's now *much*
easier to inform `wasm-bindgen` about complicated structures of types. Rest
assured though that all these descriptor functions are automatically unexported
and gc'd away, so this should not have any impact on binary sizes

A new internal trait, `WasmDescribe`, is added to represent a description of all
types, sort of like a serialization of the structure of a type that
`wasm-bindgen` can understand. This works by calling a special exported function
with a `u32` value a bunch of times. This means that when we run a descriptor we
effectively get a `Vec<u32>` in the `wasm-bindgen` CLI tool. This list of
integers can then be parsed into a rich `enum` for the JS generation to work
with.

This commit currently only retains feature parity with the previous
implementation. I hope to soon solve issues like #123, #104, and #111 with this
support.

src/closure.rs

@@ -9,6 +9,7 @@ use std::marker::Unsize;
 
 use {throw, JsValue};
 use convert::*;
+use describe::*;
 use __rt::WasmRefCell;
 
 /// A handle to both a closure in Rust as well as JS closure which will invoke
@@ -122,12 +123,20 @@ impl<T> Closure<T>
     }
 }
 
+impl<T> WasmDescribe for Closure<T>
+    where T: WasmShim + ?Sized,
+{
+    fn describe() {
+        inform(CLOSURE);
+        T::describe();
+    }
+}
+
 // `Closure` can only be passed by reference to imports.
 impl<T> ToRefWasmBoundary for Closure<T>
     where T: WasmShim + ?Sized,
 {
     type Abi = u32;
-    const DESCRIPTOR: Descriptor = T::DESCRIPTOR;
 
     fn to_abi_ref(&self, extra: &mut Stack) -> u32 {
         self.js.to_abi_ref(extra)
@@ -149,9 +158,7 @@ impl<T> Drop for Closure<T>
 ///
 /// This trait is not stable and it's not recommended to use this in bounds or
 /// implement yourself.
-pub unsafe trait WasmShim {
-    #[doc(hidden)]
-    const DESCRIPTOR: Descriptor;
+pub unsafe trait WasmShim: WasmDescribe {
     #[doc(hidden)]
     type Wrapper;
     #[doc(hidden)]
@@ -174,8 +181,9 @@ macro_rules! doit {
         ($($var:ident)*) => $arity:ident
     )*) => ($(
         // Fn with no return
-        unsafe impl<$($var: WasmAbi),*> WasmShim for Fn($($var),*) {
-            const DESCRIPTOR: Descriptor = DESCRIPTOR_FUNC;
+        unsafe impl<$($var),*> WasmShim for Fn($($var),*)
+            where $($var: WasmAbi + WasmDescribe,)*
+        {
             type Wrapper = Box<Fn($($var),*)>;
 
             fn shim() -> u32 {
@@ -209,8 +217,10 @@ macro_rules! doit {
         }
 
         // Fn with a return
-        unsafe impl<$($var: WasmAbi,)* R: WasmAbi> WasmShim for Fn($($var),*) -> R {
-            const DESCRIPTOR: Descriptor = DESCRIPTOR_FUNC;
+        unsafe impl<$($var,)* R> WasmShim for Fn($($var),*) -> R
+            where $($var: WasmAbi + WasmDescribe,)*
+                  R: WasmAbi + WasmDescribe,
+        {
             type Wrapper = Box<Fn($($var),*) -> R>;
 
             fn shim() -> u32 {
@@ -244,8 +254,9 @@ macro_rules! doit {
         }
 
         // FnMut with no return
-        unsafe impl<$($var: WasmAbi),*> WasmShim for FnMut($($var),*) {
-            const DESCRIPTOR: Descriptor = DESCRIPTOR_FUNC;
+        unsafe impl<$($var),*> WasmShim for FnMut($($var),*)
+            where $($var: WasmAbi + WasmDescribe,)*
+        {
             type Wrapper = Box<WasmRefCell<FnMut($($var),*)>>;
 
             fn shim() -> u32 {
@@ -283,8 +294,10 @@ macro_rules! doit {
         }
 
         // FnMut with a return
-        unsafe impl<$($var: WasmAbi,)* R: WasmAbi> WasmShim for FnMut($($var),*) -> R {
-            const DESCRIPTOR: Descriptor = DESCRIPTOR_FUNC;
+        unsafe impl<$($var,)* R> WasmShim for FnMut($($var),*) -> R
+            where $($var: WasmAbi + WasmDescribe,)*
+                  R: WasmAbi + WasmDescribe,
+        {
             type Wrapper = Box<WasmRefCell<FnMut($($var),*) -> R>>;
 
             fn shim() -> u32 {