Improve pretty printer performance for deep formulas and formats with long lines. Add example that demonstrates performance problem (before: 13 secs, after: 1 sec).

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add small trick to improve pretty printer performance. Now, deep.lean takes 0.140secs to be processed.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>