Add range methods to TreeMap

[Hackerpilot]

Since the TreeMap structure guarantees an ordering for its contents, it would be very useful to have front popFront, back, and popBack methods to get the largest or smallest items out of the container.

[CyberShadow]

I see that d44cb6a added front and back methods which return values, which I guess would correspond to the smallest and biggest keys.

I think for map-like types it would be better to have distinct iterator constructors, like byKey and byValue to facilitate this.