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

crates/cli-support/src/descriptor.rs

@@ -33,6 +33,7 @@ tys! {
     ENUM
     RUST_STRUCT
     CHAR
+    OPTIONAL
 }
 
 #[derive(Debug)]
@@ -59,6 +60,7 @@ pub enum Descriptor {
     Enum,
     RustStruct(String),
     Char,
+    Option(Box<Descriptor>),
 }
 
 #[derive(Debug)]
@@ -115,6 +117,7 @@ impl Descriptor {
             REFMUT => Descriptor::RefMut(Box::new(Descriptor::_decode(data))),
             SLICE => Descriptor::Slice(Box::new(Descriptor::_decode(data))),
             VECTOR => Descriptor::Vector(Box::new(Descriptor::_decode(data))),
+            OPTIONAL => Descriptor::Option(Box::new(Descriptor::_decode(data))),
             STRING => Descriptor::String,
             ANYREF => Descriptor::Anyref,
             ENUM => Descriptor::Enum,