Add disposable acquirer

[jackfirth]

Bit of a meta idea here:

(with-disposable ([acquirer disposable-acquire])
  (define foo (acquirer foo-disp))
  (define bar (acquirer bar-disp))
  ... do stuff with foo and bar ...)

Essentially, it's a disposable of an "acquirer" - a function that can be used to allocate other disposables. When the acquirer is disposed, all values allocated with it are disposed and all future attempts to call it will fail. This ended up being useful in the implementation of #96.