Add frequency count / histogram to extra::stat

[emberian]

The way I see it working is it takes an Iterator<T: Clone> and creates some sort of Map<T, uint>. It iterates over the iterator and if it finds the value in the map, bumps the count, elsewise inserts the value into the map with a count of 1.

It needs to be Clone so it can be copied into the map. There might be other traits needed depending on the map the implementation uses. At the very least probably Eq or TotalEq.

comments, @graydon?

dibs to @mihneadb

[huonw]

Added in 17c12bb.