Named binding patterns name^pattern in pattern matching, function parameters, declarations, for loops; fix complex bindings in for loops

[edemaine]

New features:

• name^pattern or name^ pattern is equivalent to pattern, but also binds the whole value to name.
  • For example, function Comp(props^{a, b}) binds props, a, and b
  • This is like name@pattern in Haskell, and pattern as name in Ocaml and Python. It's motivated by our existing {name^: pattern} syntax.
  • It works in pattern matching, function parameters, and declarations (:=, .=, const, let, var).
  • It works in = assignment, but not yet at the top level. Note a small ambiguity: x^{y} = z previously meant x ^ ({y} = z). I think this is worth overriding, as it's not useful to binary XOR with an object or array. (It's also not particularly critical at the top level, as all it's doing is making a copy of z. But I think it's cleaner and more symmetric to allow it.)
  • It works in for ... of/in loops. These were actually broken with existing @prop and splices. The following now work and did not before:
    • for @x of y
    • for [...x, y] of z
• {name^: pattern} is now shorthand for {name: name^pattern}, and it is now implemented this way.
  • As a consequence, this now works in function parameters and declarations and assignments and for loops, not just pattern matching, as suggested in Pattern name^: value binds name, while name: value never does (except for identifiers) #1663 (comment)
• In pattern matching, name < 5 now acts like the pattern < 5 but also binds the matching value to name.
  • This is especially useful for matching exceptions. We can now use try ... catch e <? RangeError to both match and get the exception e.

There's a new gatherSubbindings helper that maybe should be part of gatherBindingCode. They're now almost always used together. However, in pattern matching, they're used with somewhat different arguments, so I haven't been able to unify yet. (There's also lots of splices and thisAssignments code that's mostly duplicated in various places, so I guess this is par for the course.)

TODO: Documentation (once the feature/syntax is approved)

[STRd6]

Thanks, very nice improvements!

Seeing the examples helps me appreciate the syntax. It seems powerful and natural with room to expand in the future as well.