This commit implements the first half of [RFC rustwasm#5] where the `Deref`
trait is implemented for all imported types. The target of `Deref` is
either the first entry of the list of `extends` attribute or `JsValue`.

All examples using `.as_ref()` with various `web-sys` types have been
updated to the more ergonomic deref casts now. Additionally the
`web-sys` generation of the `extends` array has been fixed slightly to
explicitly list implementatoins in the hierarchy order to ensure the
correct target for `Deref` is chosen.

[RFC rustwasm#5]: https://github.com/rustwasm/rfcs/blob/master/text/005-structural-and-deref.md

Previously `arguments` was used to pass around an array of arguments,
but this wasn't actually a `js_sys::Array` but rather a somewhat
esoteric internal object. When switching over `Array` methods to be
`structural` this caused issues because the inherent methods on an
`arguments` object were different than that of `js_sys::Array`.

This was a copy/paste typo!

This commit removes shims, where possible, for `structural` items.
Instead of generating code that looks like:

    const target = function() { this.foo(); };
    exports.__wbg_thing = function(a) { target.call(getObject(a)); };

we now instead generate:

    exports.__wbg_thing = function(a) { getObject(a).foo(); };

Note that this only applies to `structural` bindings, all default
bindings (as of this commit) are still using imported targets to ensure
that their binding can't change after instantiation.

This change was [detailed in RFC rustwasm#5][link] as an important optimization
for `structural` bindings to ensure they've got performance parity with
today's non-`structural` default bindings.

[link]: https://rustwasm.github.io/rfcs/005-structural-and-deref.html#why-is-it-ok-to-make-structural-the-default

The wasm spec defines boolean conversion when crossing to the wasm type
i32 as 1 for `true` and 0 for `false`, so no need for us to do it
ourselves!

This commit switches all imports of JS methods to `structural` by
default. Proposed in [RFC 5] this should increase the performance of
bindings today while also providing future-proofing for possible
confusion with the recent addition of the `Deref` trait for all imported
types by default as well.

A new attribute, `host_binding`, is introduced in this PR as well to
recover the old behavior of binding directly to an imported function
which will one day be the precise function on the prototype. Eventually
`web-sys` will switcsh over entirely to being driven via `host_binding`
methods, but for now it's been measured to be not quite as fast so we're
not making that switch yet.

Note that `host_binding` differs from the proposed name of `final` due
to the controversy, and its hoped that `host_binding` is a good
middle-ground!

[RFC 5]: https://rustwasm.github.io/rfcs/005-structural-and-deref.html