def-map-type's predefined count doesn't run in constant time

[xificurC]

I think the culprit is here. (counted? (keys {:a 1})) returns false, so this is actually a lie :)

I think you cannot provide a default implementation and should instead force the users to define count themselves.

Right now defining count myself within def-map-type solves the performance issue.

[KingMob]

A valid point, but we can't break existing code. Want to submit a PR?