Rework the "type" system

This patch merges the _OP_REQP and _OP_DEFS class attributes into a
_OP_PARAMS list, which holds both. The associated unittest checks that
all opcode attributes are declared and checked, and that no LU uses the
old fields (could be removed later).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>