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/backend/src/codegen.rs

@@ -513,6 +513,10 @@ impl ToTokens for ast::ImportType {
                 }
             }
 
+            impl ::wasm_bindgen::convert::OptionIntoWasmAbi for #name {
+                fn none() -> Self::Abi { 0 }
+            }
+
             impl ::wasm_bindgen::convert::FromWasmAbi for #name {
                 type Abi = <::wasm_bindgen::JsValue as
                     ::wasm_bindgen::convert::FromWasmAbi>::Abi;
@@ -527,6 +531,10 @@ impl ToTokens for ast::ImportType {
                 }
             }
 
+            impl ::wasm_bindgen::convert::OptionFromWasmAbi for #name {
+                fn is_none(abi: &Self::Abi) -> bool { *abi == 0 }
+            }
+
             impl<'a> ::wasm_bindgen::convert::IntoWasmAbi for &'a #name {
                 type Abi = <&'a ::wasm_bindgen::JsValue as
                     ::wasm_bindgen::convert::IntoWasmAbi>::Abi;