Frozen versions of bolton's data structures

Hello!

First of all, thanks for the code!

Second, boltons introduces a FrozenDict to complement Python's own dict/frozenset, but does not introduce frozen variants of its own extensions like OneToOne (or its own IndexedSet). I wonder if this is something you would consider adding, though I can see how it is a pain two have two variants of most* data structures in the library.

*for things like queues it doesn't really make sense