MINOR: stktable: check if a type should be used as-is

MINOR: stktable: check if a type should be used as-is

stick table types now have an extra bit named 'as_is' that allows us to
check if such type should be used as-is or if it may be involved in
arithmetic operations such as counters. This can be useful since those
types are not common and may require specific handling.

e.g.: stktable_data_types[data_type].as_is will be set to 1 if the type
cannot be used in arithmetic operations.