Optimize hexWrite with an array lookup table

[chjj]

Should fix the slowdown introduced by the hash table lookups in #303

My own benchmarks seem to indicate a 10x speedup.

[chjj]

Wrote a proper benchmark. Here are the numbers:

Without this PR:

$ node perf/write-hex.js
BrowserBuffer#write(4096, "hex") x 5,697 ops/sec ±0.29% (91 runs sampled)
NodeBuffer#write(4096, "hex") x 105,578 ops/sec ±0.04% (90 runs sampled)
Fastest is NodeBuffer#write(4096, "hex")

With this PR:

$ node perf/write-hex.js
BrowserBuffer#write(4096, "hex") x 34,100 ops/sec ±0.07% (93 runs sampled)
NodeBuffer#write(4096, "hex") x 105,699 ops/sec ±0.04% (92 runs sampled)
Fastest is NodeBuffer#write(4096, "hex")

Though, on my local machine I was getting an even larger speedup.

[dcousens]

LGTM

[chjj]

I added more cases to the test. It's a bit spammy now. Let me know if you want me to reduce that.

[dcousens]

Thanks for your work @chjj