This removes the assertions that checked plugin devs didn't do anything
overly bad in favour of using mypy to ensure appropriate type-checking.
It also moves the ProgressCallback typing information to constants in
order to avoid circular imports (since constants doesn't import
anything).
Relented on the strict import of direct objects/classes for the typing
module only. Typing module components can be directly imported because
it makes the code really painful to read and write otherwise.
This is still in-line with the python style guide adopted from Google at
http://google.github.io/styleguide/pyguide.html section 2.2.
Unfortunately in tests it turned out re was faster than a python
implementation of Wu-Manber. The Wu-Manber code has been left (because
why not) but it's not really advantageous over the re builtin since
it's compiled in C.
Mike Auty