[ new ] Quantity for proof in with-clauses

[spcfox]

Description

Currently the quantity of proof in with-clause is the same as the quantity of expression. But often proofs are needed only at compile time. Example:

filterSquared : (p : a -> Bool) -> (xs : List a) ->
                filter p (filter p xs) === filter p xs
filterSquared p [] = Refl
filterSquared p (x :: xs) with (p x) proof 0 eq
  filterSquared p (x :: xs) | False = filterSquared p xs
  filterSquared p (x :: xs) | True
    = rewrite eq in cong (x ::) (filterSquared p xs)

So I suggest adding the ability to separately specify quantities for proofs.

Note that prf is currently erased in the following code, but in this version it will now be unrestricted, as will all variables for which multiplicity is not explicitly specified. This may affect existing code and probably requires discussion.

foo x with 0 (x) proof prf

Should this change go in the CHANGELOG?

• If this is a fix, user-facing change, a compiler change, or a new paper
  implementation, I have updated CHANGELOG_NEXT.md (and potentially also
  CONTRIBUTORS.md).