Use Web IDL's new-ish interface mixins concept

[romandev]

WebIDL recently introduced dedicated syntax for mixins [1]. This
replaces the existing [NoInterfaceObject] and "implements" syntax with
"interface mixin" and "includes" in the appropriate places.

This fixes #191 issue.

Test: web-platform-tests/wpt#8767

[marcoscaceres]

Friendly ping.

[saschanaz]

This is okay, but how about using partial interface mixin as the Web IDL spec suggests?

For example, instead of the common but verbose:

interface mixin GlobalCrypto {
 readonly attribute Crypto crypto;
};

Window includes GlobalCrypto;
WorkerGlobalScope includes GlobalCrypto;

you can extend the WindowOrWorkerGlobalScope interface mixin using a partial interface mixin:

partial interface mixin WindowOrWorkerGlobalScope {
 readonly attribute Crypto crypto;
};

[marcoscaceres]

@plehegar, would be a great help if you can get this merged. It's blocking removal of implements in various places.

[plehegar]

Merged. The case to use mixin can still be made separately. I just didn't want to hold the correction in the meantime.