Feature: mirror Opal only classes into CRuby

[elia]

We will add mirror classes of things only available in Opal/browser to the CRuby implementation, e.g. Promise and possibly share the implementation.

Those classes will start out under the Opal:: namespace with an option to alias them at top-level, e.g. Opal::Promise.

• Promise
• Buffer
• JS
• Console
• Native

If something is in Opal and not in CRuby, we should consider namespacing also inside Opal in the case in which official Ruby will introduce a similar class or module, e.g. the JS module was recently introduced by Ruby-WASM.

By v2.0 we should have everything in the namespace and aliased by defualt.
By v3.0 we switch the default.