Parameterize the Filter type

In preparation for introducing filtering functionality, we convert the
'Filter' type from a '*' kind to a '* -> *' kind.

This allows us to define some general properties for the filter, and
for example introduce later an easy filter compilation, etc.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>