Add 'operator' keyword

compiler/testData/loadJava/compiledKotlin/fun/OperatorKeyword.kt

@@ -0,0 +1,5 @@
+package test
+
+class Example {
+    operator fun plus(other: Example) {}
+}

compiler/testData/loadJava/compiledKotlin/fun/OperatorKeyword.txt

@@ -0,0 +1,6 @@
+package test
+
+public final class Example {
+    /*primary*/ public constructor Example()
+    public final operator fun plus(/*0*/ other: test.Example): kotlin.Unit
+}

compiler/testData/resolvedCalls/functionTypes/invokeForExtensionFunctionType.txt

@@ -5,7 +5,7 @@ fun bar(f: Int.()->Unit) {
 
 Resolved call:
 
-Resulting descriptor: fun Int.invoke(): Unit defined in kotlin.Function1
+Resulting descriptor: operator fun Int.invoke(): Unit defined in kotlin.Function1
 
 Explicit receiver kind = BOTH_RECEIVERS
 Dispatch receiver = f {[@kotlin.Extension] Function1<Int, Unit>}

compiler/testData/resolvedCalls/functionTypes/invokeForFunctionType.txt

@@ -5,7 +5,7 @@ fun bar(f: ()->Unit) {
 
 Resolved call:
 
-Resulting descriptor: fun invoke(): Unit defined in kotlin.Function0
+Resulting descriptor: operator fun invoke(): Unit defined in kotlin.Function0
 
 Explicit receiver kind = DISPATCH_RECEIVER
 Dispatch receiver = f {Function0<Unit>}

compiler/testData/resolvedCalls/functionTypes/valOfExtensionFunctionTypeInvoke.txt

@@ -9,7 +9,7 @@ interface A {
 
 Resolved call:
 
-Resulting descriptor: fun Int.invoke(): Unit defined in kotlin.Function1
+Resulting descriptor: operator fun Int.invoke(): Unit defined in kotlin.Function1
 
 Explicit receiver kind = BOTH_RECEIVERS
 Dispatch receiver = foo {[@kotlin.Extension] Function1<Int, Unit>}

compiler/testData/resolvedCalls/functionTypes/valOfFunctionTypeInvoke.txt

@@ -9,7 +9,7 @@ fun test(a: A) {
 
 Resolved call:
 
-Resulting descriptor: fun invoke(p1: Int): Int defined in kotlin.Function1
+Resulting descriptor: operator fun invoke(p1: Int): Int defined in kotlin.Function1
 
 Explicit receiver kind = DISPATCH_RECEIVER
 Dispatch receiver = foo {Function1<Int, Int>}