Tweak insertKeyExists and deleteKeyExists - #458

These new invariant tests are great! :)

      alterF
        valid:                             FAIL
          *** Failed! Falsified (after 28 tests and 26 shrinks):
          {_->[Nothing]}
          K {hash = 9, _x = A}
          fromList [(K {hash = 0, _x = A},1),(K {hash = 9, _x = B},1),(K {hash = 9, _x = A},1)]
          [Invalid (INV6_misplaced_hash 0) (SubHashPath {partialHash = 9, lengthInBits = 5})] /= [Valid]
          Use --quickcheck-replay=980663 to reproduce.
          Use -p '/Data.HashMap.Strict.alterF.valid/' to rerun this test only.

Note for myself:

I somehow couldn't reproduce the bugs above with cabal test -O0. I should look into this!

EDIT: Explained by @treeowl in #459.

Oh, I'm starting to understand now. You're shifting as you go to avoid an extra argument?

Oh, I'm starting to understand now. You're shifting as you go to avoid an extra argument?

Exactly.

It seems that right now, there are no benchmarks using this code. All benchmarks for alterF seem to rely on rules that rewrite the code to either insert or delete.

This may change with #404 though.

I can't find a significant speedup in the alter benchmarks, but not a slowdown either. The generated Core seems slightly better.