Map is used in lots of places, but HashMap is faster, at least sometimes.
It would be worth benchmarking some of the more performance critical
parts with HashMap. The uses in AnnexRead especially.

(Set and HashSet too.)

--[[Joey]]

> Note that HashMap perfomance can degrade if an attacker provides keys
> that collide. This has been used to DOS aeson parsing. (Which could
> affect a few parts of git-annex in theory; fixed in aeson-2.0.1.0). 
> So if converting to HashMap, need to consider this. --[[Joey]]
