improve signature for last

Former-commit-id: 10236c5005

assets/test_prelude.ld

@@ -141,10 +141,10 @@ fn rest {
 }
 
 fn last {
-	"Returns the last element of a list or tuple."
-	(xs) if indexed? (xs) -> base :last (xs)
-	& (xs as :list) -> base :last (xs)
-	& (xs as :tuple) -> base :last (xs)
+	"Returns the last element of an indexed value: list, tuple, or string."
+	(xs as :list) -> base :last (xs)
+	(xs as :tuple) -> base :last (xs)
+	(str as :string) -> base :last (str)
 }
 
 fn inc {