Use bytes.NewReader for deserialize when possible.

Rather than using bytes.NewBuffer, which is a read/write entity
(io.ReadWriter), use bytes.NewReader which is only a read entitiy
(io.Reader) in all cases where it is possible.  Benchmarking shows it's
slightly faster and it's also technically more accurate since it ensures
the data is read-only.

There are a few cases where bytes.NewBuffer must still be used since a
buffer with a known length is required for those instances.