Add support for optional slice types (#507)

* Shard the `convert.rs` module into sub-modules

Hopefully this'll make the organization a little nicer over time!

* Start adding support for optional types

This commit starts adding support for optional types to wasm-bindgen as
arguments/return values to functions. The strategy here is to add two new
traits, `OptionIntoWasmAbi` and `OptionFromWasmAbi`. These two traits are used
as a blanket impl to implement `IntoWasmAbi` and `FromWasmAbi` for `Option<T>`.

Some consequences of this design:

* It should be possible to ensure `Option<SomeForeignType>` implements to/from
  wasm traits. This is because the option-based traits can be implemented for
  foreign types.
* A specialized implementation is possible for all types, so there's no need for
  `Option<T>` to introduce unnecessary overhead.
* Two new traits is a bit unforutnate but I can't currently think of an
  alternative design that works for the above two constraints, although it
  doesn't mean one doesn't exist!
* The error messages for "can't use this type here" is actually halfway decent
  because it says these new traits need to be implemented, which provides a good
  place to document and talk about what's going on here!
* Nested references like `Option<&T>` can't implement `FromWasmAbi`. This means
  that you can't define a function in Rust which takes `Option<&str>`. It may be
  possible to do this one day but it'll likely require more trait trickery than
  I'm capable of right now.

* Add support for optional slices

This commit adds support for optional slice types, things like strings and
arrays. The null representation of these has a pointer value of 0, which should
never happen in normal Rust. Otherwise the various plumbing is done throughout
the tooling to enable these types in all locations.

* Fix `takeObject` on global sentinels

These don't have a reference count as they're always expected to work, so avoid
actually dropping a reference on them.

* Remove some no longer needed bindings

* Add support for optional anyref types

This commit adds support for optional imported class types. Each type imported
with `#[wasm_bindgen]` automatically implements the relevant traits and now
supports `Option<Foo>` in various argument/return positions.

* Fix building without the `std` feature

* Actually fix the build...

* Add support for optional types to WebIDL

Closes #502

guide/src/feature-reference.md

@@ -28,6 +28,7 @@ are:
 * Borrowed exported structs (`&Foo` or `&mut Bar`)
 * The `JsValue` type and `&JsValue` (not mutable references)
 * Vectors and slices of supported integer types and of the `JsValue` type.
+* Optional vectors/slices
 
 All of the above can also be returned except borrowed references. Passing
 `Vec<JsValue>` as an argument to a function is not currently supported. Strings are

guide/src/reference.md

@@ -2,20 +2,16 @@
 
 The table below provides an overview of all the types that wasm-bindgen can send/receive across the wasm ABI boundary.
 
-| Type | `T` parameter | `&T` parameter | `&mut T` parameter | `T` return value |
-|:---:|:---:|:---:|:---:|:---:|
-| `str` | No | Yes | No | Yes |
-| `char` | Yes | No | No | Yes |
-| `bool` | Yes | No | No | Yes |
-| `JsValue` | Yes | Yes | Yes | Yes |
-| `Box<[JsValue]>` | Yes | No | No | Yes |
-| `*const T` | Yes | No | No | Yes |
-| `*mut T` | Yes | No | No | Yes |
-| `u8` `i8` `u16` `i16` `u64` `i64` `isize` `size` | Yes | No | No | Yes |
-| `u32` `i32` `f32` `f64` | Yes | Yes | Yes | Yes |
-| `Box<[u8]>`  `Box<[i8]>` `Box<[u16]>` `Box<[i16]>` `Box<[u32]>` `Box<[i32]>` `Box<[u64]>` `Box<[i64]>` | Yes | No | No | Yes |
-| `Box<[f32]>` `Box<[f64]>` | Yes | No | No | Yes |
-| `[u8]` `[i8]` `[u16]` `[i16]` `[u32]` `[i32]` `[u64]` `[i64]` | No | Yes | Yes | No |
-| `&[u8]` `&mut[u8]` `&[i8]`  `&mut[i8]` `&[u16]` `&mut[u16]` `&[i16]` `&mut[i16]` `&[u32]` `&mut[u32]` `&[i32]` `&mut[i32]` `&[u64]` `&mut[u64]` `&[i64]` `&mut[i64]` | No | No | No | Yes |
-| `[f32]` `[f64]` | No | Yes | Yes | No |
-|  `&[f32]` `&mut[f32]` `&[f64]` `&mut[f64]` | No | No | No | Yes |
+| Type | `T` parameter | `&T` parameter | `&mut T` parameter | `T` return value | `Option<T>` parameter | `Option<T>` return value |
+|:---:|:---:|:---:|:---:|:---:|:---:|
+| `str` | No | Yes | No | Yes | Yes | No |
+| `char` | Yes | No | No | Yes | No | No |
+| `bool` | Yes | No | No | Yes | No | No |
+| `JsValue` | Yes | Yes | Yes | Yes | No | No |
+| `Box<[JsValue]>` | Yes | No | No | Yes | Yes | yes |
+| `*const T` | Yes | No | No | Yes | No | No |
+| `*mut T` | Yes | No | No | Yes | No | No |
+| `u8` `i8` `u16` `i16` `u64` `i64` `isize` `size` | Yes | No | No | Yes | No | No |
+| `u32` `i32` `f32` `f64` | Yes | Yes | Yes | Yes | No | No |
+| `Box<[u8]>`  `Box<[i8]>` `Box<[u16]>` `Box<[i16]>` `Box<[u32]>` `Box<[i32]>` `Box<[u64]>` `Box<[i64]>` `Box<[f32]>` `Box<[f64]`> | Yes | No | No | Yes | Yes | Yes |
+| `[u8]` `[i8]` `[u16]` `[i16]` `[u32]` `[i32]` `[u64]` `[i64]` `[f32]` `[f64]` | No | Yes | Yes | No | Yes | No |