Dict as a builtin type

[zth]

This sets dict up as a builtin type. Preparation for first classing dicts:

1. Optimize creation so it can compile to a clean instantiation of a JS object Specialize uses of new dict literal to direct JS object creation #6538
2. Dedicated syntax with spread support and more [RFC] Dedicated syntax for creating Dicts #6545

@cristianoc is this on the right track? See any immediate issues?

[cristianoc]

Looks fine.
Will also need a one liner in gentype too, similar to this line | ( (["Pervasives"; "result"] | ["Belt"; "Result"; "t"] | ["result"]), in TranslateTypeExprFromTypes for result.

[cristianoc]

there seem to be a couple of irrelevant changes, perhaps from merge/rebase

[zth]

Yes, trying to sort those now..

[zth]

Superseded by #6590