Fix the random seed used for benchmarking
Benchmark lookup
Add an NFData instance
Make numbers more reproducible, benchmarks less redundant

I thought that I didn't need to evaluate structures before benchmarking,
but boy was I wrong! For instance, lookup time "improved" from 5
microseconds to 75 nanoseconds after doing an rnf of inputs. Ouch!
Add member and notMember functions
Fix up some more docs
Add an Arbitrary instance for CritBit
Unify lookup and member into lookupWith, add findWithDefault

According to criterion, the unified code has no effect on performance.
Reorganise exports
Fix up tests
Benchmark insert
Add lookupGT