Fix bug in sexpr operator <<.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Rename Truth to Trivial, and delete Trivial macro

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Use ',' as separator for lambda

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Mark 'implicit' parameters, and move them to the beginning

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