This patch makes protoio use the new protobuf API for
marshalling/unmarshalling text protobufs, as well as extends the tests
to cover marshalling failures.
The protobuf text output is not stable/deterministic and some spaces are
added randomly, so some integration tests have to be adjusted to account
for it.
This patch adds a missing docstrings for exported identifiers, and
adjust some of the existing ones to match the standard style.
In some cases, the identifiers were un-exported after noticing they had
no external users.
Besides improving documentation, it also reduces the linter noise
significantly.
