Update to current GHC, dependency updates, stack.yaml

[robx]

Some assorted changes to get critbit to build with current GHC.

• update for Semigroup/Monoid typeclass change
• update for criterion Pure -> Benchmark type change
• selective cabal dependency updates (mostly against base, also hashable)
• include a stack.yaml that works for me
• bump cabal version number to 0.2.1.0

I'm happy to adapt any of these changes, or split them up -- whatever seems right to you.

One note is that I get some test failures (on macos). I have not managed to run the tests with the previous version, so I'm not sure whether they were introduced by some of the dependency updates.

[robx]

Meanwhile postgrest has migrated from configurator-ng to configurator-pg, so from that point of view this update is no longer necessary.

[andrewthad]

It would be great if this could be merged.