lang: opt-in UFCS — ufcs-marked fns + alias dot-dispatch, generic binding via receiver; one binding builder for plan-side generic returns

examples/0734-modules-flat-same-name-ufcs-ambiguous/a.sx

@@ -1,2 +1,2 @@
 // a.sx authors `dup` (+1). One of two distinct flat authors of `dup`.
-dup :: (x: s64) -> s64 { return x + 1; }
+dup :: ufcs (x: s64) -> s64 { return x + 1; }