Added hackish implementation for sorted bijections. This has problems (hence it is commented out):
a) Creates objects with same type as regular bijections
b) Does not implement sorted-collection-specific methods, such as rseq and subseq
c) Fails when used with `into' as the Bijection presents as IEditableCollection, but the transient method fails due to the non-editability of the underlying sorted maps which have been slid inside it

So, looks like we need a new type for sorted bijections. Also considering sorted versions of other smart maps.