recvExact for tls
Add version of conduitParser that returns Either

...instead of throwing an exception. In some complex monad transformer
stacks, it is more preferrable to keep things simple and just return a
stream of Either values, stopping at the first Left (as we don't
really know how to continue in the general case - where are we in the
input stream?).

Also made the Position types strict in their fields to make sure we
don't cause leaks in the unexpected edge cases.
Add a test case confirming operation of conduitParser