Dedicated []-related WAM instructions?

[triska]

In #3265, we see that Scryer generates instructions such as:

...
get_constant(level(shallow),[],x(1)).
get_constant(level(shallow),[],x(2)).
...
set_constant([]).
...

where GNU Prolog emits:

...
get_nil(0),
get_nil(1),
...
unify_nil,
...

i.e, GNU Prolog uses specialized instructions where Scryer uses more general ones with the parameter [], causing overhead for creating, passing, decoding and analyzing this parameter when interpreting the instructions.

Since [] occurs so frequently in Prolog code, it seems worth considering such specialized instructions also in Scryer.