Add support for new metavariable representation in the normalizer.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Relax definition of identity_stack. Fix printer for metavariable contexts.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add simplification rule to add_inst

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add simplification to add_lift

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