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/util/src/org/jetbrains/kotlin/config/LanguageVersionSettings.kt

@@ -35,6 +35,7 @@ enum class LanguageFeature(val sinceVersion: LanguageVersion?) {
     UnderscoresInNumericLiterals(KOTLIN_1_1),
     DivisionByZeroInConstantExpressions(KOTLIN_1_1),
     InlineConstVals(KOTLIN_1_1),
+    OperatorRem(KOTLIN_1_1),
 
     // Experimental features
     MultiPlatformProjects(null),