Allow returning Result from functions

This commit adds support for exporting a function defined in Rust that returns a
`Result`, translating the `Ok` variant to the actual return value and the `Err`
variant to an exception that's thrown in JS.

The support for return types and descriptors was rejiggered a bit to be a bit
more abstract and more well suited for this purpose. We no longer distinguish
between functions with a return value and those without a return value.
Additionally a new trait, `ReturnWasmAbi`, is used for converting return values.
This trait is an internal implementation detail, however, and shouldn't surface
itself to users much (if at all).

Closes #841

src/convert/impls.rs

@@ -2,10 +2,12 @@ use core::char;
 use core::mem::{self, ManuallyDrop};
 
 use convert::{Stack, FromWasmAbi, IntoWasmAbi, RefFromWasmAbi};
-use convert::{OptionIntoWasmAbi, OptionFromWasmAbi};
+use convert::{OptionIntoWasmAbi, OptionFromWasmAbi, ReturnWasmAbi};
 use convert::traits::WasmAbi;
 use JsValue;
 
+unsafe impl WasmAbi for () {}
+
 #[repr(C)]
 pub struct WasmOptionalI32 {
     pub present: u32,
@@ -370,3 +372,22 @@ impl<T: OptionFromWasmAbi> FromWasmAbi for Option<T> {
         }
     }
 }
+
+impl IntoWasmAbi for () {
+    type Abi = ();
+
+    fn into_abi(self, _extra: &mut Stack) -> () {
+        self
+    }
+}
+
+impl<T: IntoWasmAbi> ReturnWasmAbi for Result<T, JsValue> {
+    type Abi = T::Abi;
+
+    fn return_abi(self, extra: &mut Stack) -> Self::Abi {
+        match self {
+            Ok(v) => v.into_abi(extra),
+            Err(e) => ::throw_val(e),
+        }
+    }
+}