Revert "Replaced asString with JSON.stringify"

[mcollina]

This reverts commit 1a63b7c.

Fixes #17

[delvedor]

New bench:

JSON.stringify array x 3,575 ops/sec ±3.36% (85 runs sampled)
fast-json-stringify array x 2,806 ops/sec ±0.87% (94 runs sampled)

JSON.stringify long string x 13,917 ops/sec ±0.68% (90 runs sampled)
fast-json-stringify long string x 14,005 ops/sec ±0.72% (94 runs sampled)

JSON.stringify short string x 5,237,933 ops/sec ±0.76% (89 runs sampled)
fast-json-stringify short string x 11,878,885 ops/sec ±0.62% (91 runs sampled)

JSON.stringify obj x 1,862,224 ops/sec ±0.84% (94 runs sampled)
fast-json-stringify obj x 2,795,518 ops/sec ±0.67% (94 runs sampled)

LGTM.