[WIP] Add new formatter that can do better pretty-printing

[joshuawarner32]

Instead of trying to do formatting all in one go, instead build up an efficient tree structure first and only after that decide where to place newlines/etc. Notably, this let's us use relatively simple/clean heuristics for removing newlines as long as the overal visual line length remains manageable. It also removes the need for separate is_multiline/format code, where the former has to infer what the latter will do.

Highly WIP! Not ready for merging!