extensionsUtils.kt uses FuzzyType + more correct treatment of receiver nullability there

idea/testData/completion/basic/common/SubstitutedSignature5.kt

@@ -0,0 +1,7 @@
+fun<T> T.doSomething(t: T): T = t
+
+fun foo(s: String?) {
+    s.<caret>
+}
+
+// EXIST: { itemText: "doSomething", tailText: "(t: String?) for T in <root>", typeText: "String?" }

idea/testData/completion/basic/common/SubstitutedSignature6.kt

@@ -0,0 +1,7 @@
+fun<T> T.doSomething(t: T): T = t
+
+fun foo(s: String?) {
+    s?.<caret>
+}
+
+// EXIST: { itemText: "doSomething", tailText: "(t: String) for T in <root>", typeText: "String" }