Non-end rest parameters support types

[edemaine]

Fixes #929 by

• Typing new end rest parameter as tuple type
• Add type casting of splice return value
• Add ref to type cast remaining array, but only for TypeScript output

Lots of other internal cleanup along the way:

• Fix traversal functions to allow non-Predicate arguments instead of erroring. In this case, they just don't filter.
• Move NonEmptyParameters handler into processParams
• Uniform use of PostRestBindingElements in blockPrefix
• Parameters node now has parameters property with array of actual parameters
• Internal type fixes

[STRd6]

Thanks, great improvement! I’m glad the TS types ended up being so simple.