Add isolate_pairs() function.

Generate a list of base pair indices which are 'isolated'.
The bases to the left and right of the bases in question are not
viable edges (i.e. cannot be joined).
Add function recursive_reduce.

Function recursively eliminates isolated base pairs.
Returns a string with all eliminated isolated base pairs.
Add seto

This is a working version.
Add dbru.

This is a working version.
Add trie.

Working version.