Oid: remove Bytes()

This is not needed. We can do id[:] to get a slice.
Oid: use Go's conversion functions

Go already has all the necessary pieces for encoding and decoding hex
strings. Using them let's us avoid going into C land.

Benchmarks show this takes about half the time as using libgit2's
functions.
Oid: fix IsZero()

We need to compare against the number zero, not its ASCII value.