[Tolk] Smart casts and control flow graph

With the introduction of nullable types, we want the
compiler to be smart in cases like
> if (x == null) return;
> // x is int now
or
> if (x == null) x = 0;
> // x is int now

These are called smart casts: when the type of variable
at particular usage might differ from its declaration.

Implementing smart casts is very challenging. They are based
on building control-flow graph and handling every AST vertex
with care. Actually, I represent cfg not a as a "graph with
edges". Instead, it's a "structured DFS" for the AST:
1) at every point of inferring, we have "current flow facts"
2) when we see an `if (...)`, we create two derived contexts
3) after `if`, finalize them at the end and unify
4) if we detect unreachable code, we mark that context
In other words, we get the effect of a CFG but in a more direct
approach. That's enough for AST-level data-flow.

Smart casts work for local variables and tensor/tuple indices.
Compilation errors have been reworked and now are more friendly.
There are also compilation warnings for always true/false
conditions inside if, assert, etc.

tolk-tester/tests/invalid-typing-24.tolk

@@ -0,0 +1,16 @@
+fun getNullableInt(): int? { return 5; }
+
+fun getTensor(x: int?): (int?, int) { return (x, 0); }
+
+fun testSmartCastsDropAfterAssign() {
+    var x: int? = 0;
+    var y: int? = 0;
+    (getTensor(x = getNullableInt()).0, getTensor(y = getNullableInt()).0) = (x + y, x - y);
+    return x+y;
+}
+
+/**
+@compilation_should_fail
+@stderr can not apply operator `+` to `int?` and `int?`
+@stderr x + y, x - y
+ */