Remove binary.{Read,Write} in favour of doing it by hand

The encoding/binary.{Read,Write} methods use reflection which is slow.
This is ok for prototyping but has a serious performance impact.  In
each case, we know exactly what we're trying to write out, so we'll
allocate our own buffer and do the explicit conversion ourselves.

As an example of the performance improvement this makes, here are the
results from an (internal) benchmark:

benchmark          old ns/op    new ns/op    delta
BenchmarkDecode       427190       318845  -25.36%
Merge pull request #5 from dgryski/remove-binary-read-write

Remove binary.{Read,Write} in favour of doing it by hand