Refactor recursive assembly index search

[Garrett-Pz]

Refactors the recurse_index_search function by creating two new structs, contained in matches.rs and state.rs

Matches Struct

• Manages the matches list of non-overlapping, isomorphic subgraph pairs.
• Generation of matches now happens when creating a new instance of this struct.
• Provides the list of matches to be tested for removal in a state through the valid_matches() function.

State Struct

• Holds and manages the data local to a state in the search space.
• After removing a duplicate subgraph match, a child state can be created using the update() function.

[jdaymude]

A full benchmark indicates that the refactor did not statistically change performance (as expected).