Add joinNullable
Add basic test cases
Make the tests into a 'cabal test' suite.
Notes on testing
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'