Improve performance of hex type decoding
Improve performance of hex type decoding

We replace the use of RegExps with Byte comparisons which should be
much faster.

It's also simpler to read IMO since we're already using Byte slice
indexes in related code.

Finally, it's actually more precise as it:
 1. Allows for X'1' and x'1' (MySQL accepts both)
 2. Requires x0<byte> as MySQL does not accept simply '0x' (it does
    accept x'' though)

Signed-off-by: Matt Lord <5c2d6cb5cc654501c0d86e59cab3211bbda644af@gmail.com>

vitessio/vitess,mahak/vitess,vitessio/vitess,vitessio/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,mahak/vitess,mahak/vitess,mahak/vitess,vitessio/vitess,mahak/vitess,vitessio/vitess