Improve fmt for comma-separated lists

[adlerjohn]

For single-line comma-separated lists (e.g. the fields of a struct) forc fmt currently:

1. Only supports formatting to a single line, and
2. Adds a whitespace at the end of the last element in the list, after the comma.

It should support certain comma-separated lists always being one element per line (e.g. struct fields will always be one per line, no one writes them in a single line), which should also fix the abovementioned bug.