lint(*): curly braces linter (#10280)
This PR:
1. Adds a style linter for curly braces: a line shall never end with `{` or start with `}` (modulo white space)
2. Adds `scripts/cleanup-braces.{sh,py}` to reflow lines that violate (1)
3. Runs the scripts from (2) to generate a boatload of changes in mathlib
4. Fixes one line that became to long due to (3)
