Effects: add diagnostic tests on contracts

- Make AbstractDiagnosticsTest dump function contracts
- Add diagnostics tests on parsing contracts
- Add diagnostics tests on smartcats in presence of functions with
contracts
- Add diagnostics tests on initialization and flow in presence of
in-place called lambdas

==========
Introduction of EffectSystem: 16/18

compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts/receiver.kt

@@ -0,0 +1,34 @@
+// !LANGUAGE: +ReturnsEffect
+// !DIAGNOSTICS: -INVISIBLE_REFERENCE -INVISIBLE_MEMBER
+
+import kotlin.internal.contracts.*
+
+fun Any?.isNull(): Boolean {
+    contract {
+        returns(false) implies (this@isNull != null)
+    }
+    return this == null
+}
+
+fun smartcastOnReceiver(x: Int?) {
+    if (x.isNull()) {
+        x<!UNSAFE_CALL!>.<!>inc()
+    }
+    else {
+        <!DEBUG_INFO_SMARTCAST!>x<!>.dec()
+    }
+}
+
+class UnstableReceiver {
+    var x: Int? = 42
+
+    fun smartcastOnUnstableReceiver() {
+        if (x.isNull()) {
+            x<!UNSAFE_CALL!>.<!>inc()
+        }
+        else {
+            <!SMARTCAST_IMPOSSIBLE!>x<!>.dec()
+        }
+    }
+}
+