Add support for $_BUF_ cell type in Yosys import.

[brianhuffman]

Also do a bunch of code style changes and refactorings.

Fixes #2828.

Note that because this branch does a lot of code reformatting and reindentation that touches a huge number of lines of code, it is probably easiest to review by looking at individual commits.

[RyanGlScott]

Is it reasonable to add a test case for this?

[RyanGlScott]

This adds just one of the cell types listed here. Do you foresee the need to add support for the others? (If so, I'd be fine with deferring that to a separate issue.)

[brianhuffman]

It really depends on what kinds of output we see from our synthesis tools. We could go ahead and implement all of the gate types listed in the Yosys cell library, but if we don't have any generated circuits that use a particular gate type, then we can't really test the implementation properly. It might be the case that $_BUF_ is the only additional gate type that TabbyCAD ever adds, and if so it might be best to hold off on implementing the others.

[brianhuffman]

Yes, we should definitely add a test case for this. The thread for #2828 doesn't include a proof script, but it says it's based on an example from a tutorial, so I should be able to put something together.

[sauclovian-g]

FWIW, I rather prefer the do positioning the other way. Which is not to say you shouldn't change it here. But maybe we should talk about it offline...

[podhrmic]

With this patch, I can indeed load the TabbyCAD generated adder.json file from the turorial, thanks!

[RyanGlScott]

For what it's worth, the already-existing test_yosys_adder test case essentially just checks in adder_yosys.json from #2828 (comment). For testing purposes, I think it would suffice to use adder_tabby.json from that comment.

[brianhuffman]

I added a new test case that loads both .json files and proves that the add4 functions from each are the same. This ensures not only that $_BUF_ is imported without error, but that it is given the correct semantics.