Explicitly add list pattern when matching on non-empty list on the Erlang target

[giacomocavalieri]

From Erlang's efficiency guide:

Don't:

map_pairs1(_Map, [], Ys) -> Ys;
map_pairs1(_Map, Xs, []) -> Xs;
map_pairs1(Map, [X|Xs], [Y|Ys]) -> todo.

Do:

map_pairs2(_Map, [], Ys) -> Ys;
map_pairs2(_Map, [_|_]=Xs, [] ) -> Xs;
map_pairs2(Map, [X|Xs], [Y|Ys]) -> todo.

In the second example, since Xs can no longer match with anything, the compiler is free to reorder the branches and make it as efficient as possible. I wonder if the Gleam compiler could emit code that plays nicely with this when it can tell you're matching on a non-empty list.

[lpil]

Sounds good! As a small perf optimisation let's not do this right now unless it can be implemented very easily and quickly. Or if anyone suuuper wants to do it 😁