constrain hashable and fix naming conflict

Two small changes:

1. We have to constrain `hashable` to a pre-1.2 version since 1.2 changes the api. In theory we should constrain all of our dependencies to major versions so things like hashable-1.2 don't affect newcomers.

2. rename `tests/pos/zipw.hs` to `tests/pos/zipW2.hs` to avoid a naming conflict on case-insensitive filesystems, like HFS+.