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/codegen/box/operatorConventions/remAssignmentOperation.kt

@@ -0,0 +1,17 @@
+class A() {
+    var x = 5
+}
+
+operator fun A.modAssign(y: Int) { throw RuntimeException("mod has been called instead of rem") }
+operator fun A.remAssign(y: Int) { x %= y + 1 }
+
+fun box(): String {
+    val original = A()
+    val a = original
+
+    a %= 2
+    if (a !== original) return "Fail 1: $a !== $original"
+    if (a.x != 2) return "Fail 2: ${a.x} != 2"
+
+    return "OK"
+}

compiler/testData/codegen/box/operatorConventions/remOverModOperation.kt

@@ -0,0 +1,18 @@
+class A() {
+    var x = 5
+
+    operator fun mod(y: Int) { throw RuntimeException("mod has been called instead of rem") }
+    operator fun rem(y: Int) { x -= y }
+}
+
+fun box(): String {
+    val a = A()
+
+    a % 5
+
+    if (a.x != 0) {
+        return "Fail: a.x(${a.x}) != 0"
+    }
+
+    return "OK"
+}