overriding standard types

I tried to add `haxe.io.Bytes` implementation, but there's a problem that in a macro it also tries to use the overriden class and then complains about using `js.node.Buffer` in a macro context.

What can we do with that? One option is to just move nodejs sys stuff into Haxe itself (and fence with `#if nodejs`), another one is to make it possible to override types only for target output context, not the macro one.

Also, `@:coreApi` metadata doesn't work on our overriden types.

cc @Simn @ncannasse