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

guide/src/SUMMARY.md

@@ -30,6 +30,7 @@
     - [`String`](./reference/types/string.md)
     - [Number Slices](./reference/types/number-slices.md)
     - [Boxed Number Slices](./reference/types/boxed-number-slices.md)
+    - [`Result<T, JsValue>`](./reference/types/result.md)
   - [`#[wasm_bindgen]` Attributes](./reference/attributes/index.md)
     - [On JavaScript Imports](./reference/attributes/on-js-imports/index.md)
       - [`catch`](./reference/attributes/on-js-imports/catch.md)

guide/src/reference/types/result.md

@@ -0,0 +1,20 @@
+# `Result<T, JsValue>`
+
+| `T` parameter | `&T` parameter | `&mut T` parameter | `T` return value | `Option<T>` parameter | `Option<T>` return value | JavaScript representation |
+|:---:|:---:|:---:|:---:|:---:|:---:|:---:|
+| No | No | No | No | No | Yes | Same as `T`, or an exception |
+
+The `Result` type can be returned from functions exported to JS as well as
+closures in Rust. Only `Result<T, JsValue>` is supported where `T` can be
+converted to JS. Whenever `Ok(val)` is encountered it's converted to JS and
+handed off, and whenever `Err(error)` is encountered an exception is thrown in
+JS with `error`.
+
+You can use `Result` to enable handling of JS exceptions with `?` in Rust,
+naturally propagating it upwards to the wasm boundary. Furthermore you can also
+return custom types in Rust so long as they're all convertible to `JsValue`.
+
+Note that if you import a JS function with `Result` you need
+`#[wasm_bindgen(catch)]` to be annotated on the import (unlike exported
+functions, which require no extra annotation). This may not be necessary in the
+future though and it may work "as is"!.