Add let and heterogeneous equality. Add bool_type and bool_value.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Rename eqp --> is_eqp. The name is too similar to heterogeneous equality constructor eq.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
Add normalization a = b for values (aka semantic attachments)

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

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