Add head_reduce_mmv (reduction function modulo metavariables)

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Fix is_convertible propositions => type

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Refactor elaborator using new metavar library.

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