perf: use set

[jeswr]

Uses a set rather than object for the final layer of the index.

Short term performance gain from being able to use built-in set operations.

Long term gain from being able to use symbols for indicies

First tests seem to indicate a performance degredation

main

$ node perf/N3Store-perf.js 
N3Store performance test
- Adding 16777216 triples to the default graph: 23.328s
* Memory usage for triples: 855MB

This branch

$ node perf/N3Store-perf.js 
N3Store performance test
- Adding 16777216 triples to the default graph: 39.701s
* Memory usage for triples: 1083MB

[RubenVerborgh]

Sweet, I like the idea 👍

[RubenVerborgh]

RE: degradation, know that the benchmark might have a flawed design. It's an atypical insertion pattern. Retrieval is typically much more common than insertion.

Could be an option to have a switchable implementation where different read/write performance balance is desired.