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/light-analysis/operatorConventions/remAssignmentOperation.txt

@@ -0,0 +1,13 @@
+public final class A {
+    private field x: int
+    public method <init>(): void
+    public final method getX(): int
+    public final method setX(p0: int): void
+}
+
+
+public final class RemAssignmentOperationKt {
+    public final static @org.jetbrains.annotations.NotNull method box(): java.lang.String
+    public final static method modAssign(@org.jetbrains.annotations.NotNull p0: A, p1: int): void
+    public final static method remAssign(@org.jetbrains.annotations.NotNull p0: A, p1: int): void
+}

compiler/testData/codegen/light-analysis/operatorConventions/remOverModOperation.txt

@@ -0,0 +1,13 @@
+public final class A {
+    private field x: int
+    public method <init>(): void
+    public final method getX(): int
+    public final method mod(p0: int): void
+    public final method rem(p0: int): void
+    public final method setX(p0: int): void
+}
+
+
+public final class RemOverModOperationKt {
+    public final static @org.jetbrains.annotations.NotNull method box(): java.lang.String
+}