Keyword Rest implementation

[AlexKnauth]

An implementation of https://github.com/racket/rhombus-prototype/pull/225 option A: Rests with & and ~&, Repetitions with ..., Strings with +&.

Things left to do:

• [x] Implement keyword-apply-like function calls with ~& keyword rest
• [x] Document new features
• [ ] Test ~& keyword rest thoroughly
• [x] Implement & rest for more data constructors
  • [x] List exprs/binds with [e, ..., & l] and List(e, ..., & l)
  • [x] Map exprs/binds with {k: v, ..., & m}
  • [x] Map exprs/binds with Map{k: v, ..., & m}
  • [x] Map expressions with Map([k, v], ..., & l)
  • [x] Set expressions with {e, ...+, & s}
  • [x] Set expressions with Set{e, ...+, & s}
  • [x] Set expressions with Set(e, ...+, & l)
• [x] Implement empty map binding, usable for something like {k: v, ..., & Map.empty}
• [x] Resolve issues with Map() and & rest not working the same way as {} and & rest (Map{} macro, Map() function with keys/values grouped together instead of alternating, no & rest in Map() binding for now, but with the possibility of resolving that in the future via entry-sequences suggested by @jackfirth)
• [x] Resolve issues with matching & rest of mutable data (~error, pattern-fail~, copy, ~or alias-slice?~)
• [x] Implement ~& keyword rest for | alt functions
• [x] Implement keyword arguments for | alt functions, cooperating with ~& keyword rest
• [x] Check raco expand on test cases like the one below with (#%app f temp8 temp7)
• [ ] Test everything thoroughly
• [ ] Clean up code that ends up unused
• [x] Rebase / Fix merge conflicts

From the Rhombus meeting on 2022-08-18:

#lang rhombus

fun f(x, ~b):
  b

fun g():
  f(1, ~b: 2)

the expansion for g should ideally contain something like (if (#%app variable-reference-constant? (#%variable-reference f6)) (#%app f temp8 temp7) ....) in its body.