Introduce "use expression body" inspection #KT-16063 Fixed

Converted from the relevant intention
Reported cases: one-liners, whens
Also, more exact caret detection in local inspection tests

idea/testData/inspectionsLocal/useExpressionBody/convertToExpressionBody/assignPlus.kt

@@ -0,0 +1,7 @@
+// PROBLEM: none
+
+var x = 0
+
+fun <caret>foo() {
+    x += 1
+}