Preliminary declaration visitor for estimating local variable's predictability for smart casts

Predictability estimation algorithm is completely new, but backward compatibility should present.
A large set of tests. Some updated tests.
Smart casts allowed for captured variables if they are not modified in closure #KT-9051 Fixed
Also #KT-8643 Fixed
Also #KT-7976 Fixed
Correct handling of lambda arguments in functions #KT-9143 Fixed

compiler/testData/diagnostics/testsWithStdLib/smartcasts/letChangesToNotNull.kt

@@ -0,0 +1,7 @@
+// KT-9051: Allow smart cast for captured variables if they are not modified
+
+fun foo(y: String?) {
+    var x: String? = null
+    y?.let { x = it }
+    x<!UNSAFE_CALL!>.<!>length // Smart cast is not possible
+}