[skrifa] autohint: schedule a cleanup pass

[dfrg]

Initial implementation was focused on matching FreeType both for expediency and to ensure correctness. There is a lot of room for improvement in code quality by making use of more idiomatic Rust code and adding methods/types for common patterns.

For example: #1134 (comment) and #1134 (comment)

Also try to document more of the magic numbers. ref #1145 (comment)