Improve serialization perf

[zertosh]

The "Serializing source map" benchmark time was cut in half. About half of those gains are from reducing the calls to util.toSetString in ArraySet, the other half is from removing the duplicate calls to indexOf.

Before:

❯ node bench-shell-bindings.js
Parsing source map
iteration: 5080ms
iteration: 4665ms
iteration: 5053ms
iteration: 4730ms
iteration: 4986ms
{ unit: 'ms', x2: 120325071, x1: 24513, x0: 5 }

Serializing source map
iteration: 8028ms
iteration: 8019ms
iteration: 7959ms
{ unit: 'ms', x2: 192098826, x1: 24006, x0: 3 }

After:

❯ node bench-shell-bindings.js
Parsing source map
iteration: 4951ms
iteration: 4692ms
iteration: 4969ms
iteration: 4765ms
iteration: 5050ms
{ unit: 'ms', x2: 119416422, x1: 24426, x0: 5 }

Serializing source map
iteration: 3915ms
iteration: 3843ms
iteration: 3778ms
iteration: 3766ms
iteration: 3943ms
iteration: 4147ms
{ unit: 'ms', x2: 91296772, x1: 23392, x0: 6 }

[fitzgen]

Looks great! Thanks! I'm just going to run the benchmarks locally, real quick.

[fitzgen]

Very nice :)

[zertosh]

np @fitzgen! I just rebased #202, any thoughts on it?