[kosha] Refactor `Kosha` logic into a new `MultiFst` class

[akprasad]

MultiFst should handle duplicate keys and do mst of the heavy lifting. Kosha should then be a thin wrapper over MultiFst.

Use cases:

• can use MultiFst for other kinds of linguistic data
• can create Python bindings directly for MultiFst which can be handy for some applications (e.g. creating lightweight FST structs for word lists.)

Technical blockers

• to avoid double allocation, get_all should return an iterator instead of a Vec.