Revert "Sped up hashify" because it causes memoization cache collisions.

This reverts commit 9495a6b816fd6101acadb06fb0bb899b06e3a87c.
Removed useage of BigInt in memoization that was related to otherHashCode
Added a constant in hashify so single-element collections don't collide with their contents.
Various hashing changes to prevent collisions and some import cleanup.
Merge branch 'hash-fix'
Merge pull request #220 from heiseda/master

Hash code fixes and improvements