Performance issues when adding large number of entities

[fbacall]

See: https://github.com/kit-data-manager/ro-crate-benchmarks

[fbacall]

Probably due to how entities are looked up by iterating through an array, rather than using some kind of hash map

[fbacall]

^ cuts time to add 500 entities from 7.5 secs to ~0.4 secs on local machine