prefer 'HashMap' to 'BTreeMap' when ordering isn't important

[danieleades]

not sure why BTreeMap has been used instead of HashMap here, given that there's no need to preserve ordering (is there?).

I've also removed some allocations by using &'static str for the keys instead of String. Should provide some (very) modest performance gains.

shame the benchmarks aren't working... *cough* #164 *cough*