Implement "Fix compacting property-based indexes" from json-ld-api

[RoboPhred]

An issue was identified with the json-ld spec in json-ld-api issue 514 where property indexes are not processed correctly, and all properties end up under "@none". This was fixed with an algorithm change in json-ld-api PR 528.

jsonld.js (and the playground) still seem to exhibit this issue, with a playground example given in the original issue:
Playground link