Implement hash-based lookup and creation for archetypes

[ImmutableOctet]

Archetypes are currently mapped using standard containers, where the file path is stored as the key to the processed entity descriptor. We should change the key to only be a hash of the archetype path, or have a separate mechanism for finding entries based on the string hash.

The creation mechanism mentioned in #74 would be made more efficient with this change as we wouldn't need to store a string for every creation request.