Infrastructure for binary parameter formats

Work towards #7.
Boolean parameters transmitted as binary

Work towards #7.
Binary transmission of numeric types

It looks like Postgres doesn't guarantee that its floats are IEEE754,
but I don't know if there are any reasonable platforms on which the
format is actually different.

Work towards #7.
Batch command sequence writes

This allows command sequences like Prepare/Describe/Sync to be sent in
fewer TCP packets.