arenas: "colored function" and "duplicate implementation" problems
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Functions that allocate in arenas versus functions that allocate owned
references is a kind of "colored function" problem, and also a "duplicate
implementation" problem.

What would be really nice to do is:
1. Allow functions that work with (take and return) arenas
   to also work on a special "pseudo arena" that instead
   allocates with standard malloc/free-style functions.
   --> This would solve the "duplicate implementation" problem.
2. Allow functions that take only owned/non-arena references to somehow
   also work with arenas (and call suchs functions).
   --> This would solve the "colored function" problem.

Possible solutions:
- Use a thread local variable to hold the arena pointer.
  (How can we use multiple arenas in one function in that case?)
- Add separate parameters for the arenas.
- "Lightweight arenas"? (see lightweight_arenas.txt)
- more...?

Partial solutions:
- For "arena any followme" style parameters, we could accept a NULL parameter
  to allocate with malloc/free-style functions.


Tricky problem:
- arena allocation and malloc/free-style allocation is VERY different, and
  cannot typically be merged into one implementation that supports both.
  --> We could add a separate "either_arena_or_own" keyword for the basic
      cases where it would work.