Define divides, and add examples

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Define absolute value function and notation for it. Add new example.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Move examples to lean examples subdir. We should have an example subdirectory for each frontend

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