json: Add benchmark (+ fix existing benchmarks)

[ibgreen]

Adds benchmarks for JSON, comparing to JSON.parse. Also adds an (unfortunately unfavorable) comparison to d3-csv for the CSVLoader

CSV Decode
├─ CSVLoader#parse: 279 iterations/s ±3.77%
├─ d3-dsv#parse: 2.13K iterations/s ±2.57%

Clarinet - JSON parsing from string
├─ JSON.parse (used by parseSync): 26.6K iterations/s ±3.55%
├─ ClarinetParser (used by parseInBatches): 7.15K iterations/s ±1.64%

JSONLoader - loading from file
├─ load(JSONLoader) - Uses JSON.parse: 717 iterations/s ±6.61%
├─ loadInBatches(JSONLoader) - Uses Clarinet: 204 iterations/s ±19.31%

[coveralls]

Coverage increased (+3.4%) to 53.63% when pulling 286692e on ib/json-bench into 140fd02 on master.

[xintongxia]

why do we need this generators here?

[ibgreen]

This benchmark compares the speed of non-async iteration with async iteration.

Async iteration is significantly slower.