Extraction Engine: Generate function/property accessor with expression body (whenever applicable)

#KT-6405 Fixed

idea/testData/refactoring/extractFunction/duplicates/insertAfterDuplicates.kt.after

@@ -3,7 +3,5 @@ class A {
     val x = i()
     val y = i()
 
-    private fun i(): Int {
-        return 1 + 1
-    }
+    private fun i() = 1 + 1
 }

idea/testData/refactoring/extractFunction/duplicates/insertBeforeDuplicates.kt.after

@@ -4,6 +4,4 @@ fun foo() {
     val y = i()
 }
 
-private fun i(): Int {
-    return 1 + 1
-}
+private fun i() = 1 + 1

idea/testData/refactoring/extractFunction/duplicates/nestedStrongMatch.kt.after

@@ -5,6 +5,4 @@ fun main(args: Array<String>) {
     val y = i()
 }
 
-private fun i(): Int {
-    return 1 + 1
-}
+private fun i() = 1 + 1

idea/testData/refactoring/extractFunction/duplicates/notNullAssertion.kt.after

@@ -11,6 +11,4 @@ fun foo(): Int {
     }
 }
 
-private fun i(s: String?): Int {
-    return s!!.length
-}
+private fun i(s: String?) = s!!.length

idea/testData/refactoring/extractFunction/duplicates/singleExpression.kt.after

@@ -24,6 +24,4 @@ fun bar() {
     i(f(), f())
 }
 
-private fun i(a: Int, b: Int): Int {
-    return a + b * a
-}
+private fun i(a: Int, b: Int) = a + b * a