Introduce mem-rel! function in builtin_stdlib.elpi

The motivation is that if `f` is of type `o:A, i:B`, then `mem-rel! A f Bs` is a bit more readable than `mem! B {{map As {{swap f}} }}`.

for example,
```
pred f i:term i:prop.
f C (decl C _ _).
f C (def C _ _ _).

mem-rel! C f Ctx.
```


Not sure what the name should be.

Author: patrick-nicodemus

src/builtin_stdlib.elpi

@@ -201,7 +201,10 @@ mem [_|L] X :- mem L X.
 
 func exists! list A, (pred A).
 exists! [X| _] P :- P X, !.
-exists! [_|XS] P :- exists! XS P. 
+exists! [_|XS] P :- exists! XS P.
+
+func mem-rel! A, (pred A B), list B.
+mem-rel! A R L :- exists! L (B\ R A B).
 
 pred exists list A, (pred A).
 exists [X|_] P :- P X.
@@ -318,4 +321,4 @@ max _ M M.
 func findall prop -> list prop.
 findall P L :- findall_solutions P L.
 
-}
+}