Allow contravariant 1.2.*

So many bumps!
Merge pull request #69 from bergmark/patch-1

Allow contravariant 1.2.*
Unpackspec pc boilerplate
ExprArr: add Num instance.

Makes writing where/restrict expressions much nicer.
cast
Dubious QueryRunner instance for nested Maybes
ExprArr: lower, coalesce, cast
Drop haskelldb constraint
Merge remote-tracking branch 'hesselink/implement-offset'