Switch all imports to structural by default

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

tests/wasm/host_binding.js

@@ -0,0 +1,25 @@
+const assert = require('assert');
+
+exports.MyType = class {
+  static foo(y) {
+    assert.equal(y, 'x');
+    return y + 'y';
+  }
+
+  constructor(x) {
+    assert.equal(x, 2);
+    this._a = 1;
+  }
+
+  bar(x) {
+    assert.equal(x, true);
+    return 3.2;
+  }
+
+  get a() {
+    return this._a;
+  }
+  set a(v) {
+    this._a = v;
+  }
+};