PermitPost :postbox: -> Permit2 :two: (#52)

* feat: permitpost -> permit2

* feat: compiling with permit2 but tests failing

* feat: tests passing now

* feat: flatten typehash to save some keccaks

* feat: cleanup hashing logic

Hashing logic is ugly and big and only gets called in one spot. This
commit cleans it up by moving it into separate libraries per ordertype
to keep the concrete reactor code readable

* feat: helper for hashing single order

* fix: comment

* fix: merge issues with input decay

* fix: latest permit2

* feat: sara comments

* fix: readme

* fix: move order array hash to helper