Gradual migration of operator 'mod' to 'rem'

- Introduce new 'rem' operator convention
 - Prefer 'rem()' to 'mod()' when both are available, even if mod() is a
   member, and rem() -- an extension
 - Place operator 'rem' under the language feature

compiler/testData/diagnostics/tests/operatorRem/preferRemAsExtentionOverMod.kt

@@ -0,0 +1,12 @@
+// !DIAGNOSTICS: -UNUSED_PARAMETER
+
+class Foo {
+}
+operator fun Foo.mod(x: Int): Foo = Foo()
+operator fun Foo.rem(x: Int): Int = 0
+
+fun foo() {
+    takeInt(Foo() % 1)
+}
+
+fun takeInt(x: Int) {}