add test for public, update changelog

changelogs/unreleased/1224-dark64

@@ -1 +1 @@
-Disallow the use of the `private` keyword on non-entrypoint functions
+Disallow the use of the `private` and `public` keywords on non-entrypoint functions

zokrates_cli/examples/compile_errors/non_main_public_args.zok

@@ -0,0 +1,8 @@
+def mul(public field a) -> field { // `public` should not be allowed here
+    return a * a;
+}
+
+def main(field a, field b) {
+    assert(mul(a) == b);
+    return;
+}