FEE support?

[ctrlcctrlv]

We all know that FEE's implementation in fontFeatures is very slow.

Does @simoncozens think that feaparser.rlib should also do any FEE-like input transformations? Should it allow people to add Rust callbacks to the parser for arbitrary tag names not in the FEA standard?

Or is fontFeatures, although slow, good enough? Should we instead focus on porting the slow parts of Python fontFeatures to Rust?