remove calls binary.Read/Write

Read/Write use reflection and are slow.  They're not needed in this case
because we know we're always doing a uint64.  Replace them with
BigEndian.{PutUint64,Uint64} instead.
remove zeroPad.

It's dead code now that we're not using binary.Read and binary.Write.
Also remove the associated tests.
README: Test coverage is now at 100.

We don't need the section explaining otherwise.