Summary:
Using already-available lookup table instead of a bunch of branching, this improves read throughput from ~750MB/s to 2.1GB/s in benchmarks.

```
before:        (time)   (rate)
BM_unhexlify   1.39ns  719.26M
after:
BM_unhexlify 470.59ps    2.13G
```

Reviewed By: philippv

Differential Revision: D4201352

fbshipit-source-id: 8393e066c45c402ccb8b537fdb25417e8e6a9511