Refactor into compilers

[simoncozens]

This is a large refactor. We have, in init.py, a bunch of very similar functions which differ only slightly, mostly in terms of default parameters and small pieces of logic. This means (a) there's a lot of repetition, (b) there's a lot of horrible mangling of parameters, and (c) it's hard to see the small pieces which are different because of the repetition.

To mitigate this up to this point, we have essentially tried to re-create inheritance by having these functions call things like call_preprocessor, call_postprocessor (abstracting out what we can of the commonalities), and having ad hoc specialisation of the function arguments. In other words, we have tried to turn our functions into pseudo-classes. It turns out to be much neater to turn them into real classes instead.

Turning them into classes saves all the repetition, removes around 800 lines of code, makes it easier to actually find things, and possible also has exposed a bug in compileVariableCFF2s! (It's still a good idea.)

[anthrotype]

in general i'm supportive of this, as long as it doesn't unnecessarily breaks existing code that uses ufo2ft. Since these are lots of changes, it'd take me a while to review. But thanks Simon!

[anthrotype]

removes around 800 lines

it still adds another 789 ;)

[simoncozens]

Good point, I missed a directory when counting. :-)

[anthrotype]

can we make this new compilers subpackage private (e.g. prefix with underscore _compilers)? I wouldn't like to expose these new classes but prefer we keep the existing interface.

[anthrotype]

maybe we can stop fiddling with this and start merging, so the other PRs can be rebased off it, we can fix up remaining issues if any before release

thanks for patience!