This refactoring also uncovered some kind of bug, that got masked by the
`twtxt.models.Tweet` constructor, which did the actual conversion to
seconds precision. Without that Tweet container in between anymore, two
precision-related tests failed. So this precision handling needed to be
replicated into the hash creation – where it acutally belongs. One day,
we're finally getting rid of the `twtxt.models.Tweet` entirely, so it's
not really duplicated anymore.

This is part of surgically removing the twtxt reference implementation
of tt.