Could `giveme`s be passed around via some context object?

The idea would be to avoid having to pass around every single `giveme`
object to every single function that might need it (now or in a future
version).

Perhaps it could be a per-arena setting, where by default nothing is
passed, but specific `giveme`s can be allowed (and then that would be
inherited to sub-arenas).

Problem:

1. Isn't this a way to bypass pure-ness of functions?
   If a function `f` calls a function `g` several times with the same
   input, it would be expected to return the same value each time.
   But with implicit context objects, it could do anything (including
   I/O)...