[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/indexed-access.tolk

@@ -178,7 +178,7 @@ fun test114(f: int, s: int) {
 @method_id(115)
 fun test115() {
     var y = [[[[true]]]];
-    return (y, y.0.0.0.0 = !y.0.0.0.0, y.0);
+    return (y, ((((y).0).0).0).0 = !y.0.0.0.0, y.0);
 }
 
 @method_id(116)
@@ -248,7 +248,7 @@ fun test122(x: (int, int)) {
 @method_id(123)
 fun test123() {
     var t = [[10, 20]] as [[int,int]]?;
-    t!.0.0 = t!.0.1 = 100;
+    ((t!).0).0 = ((t!).0).1 = 100;
     return t;
 }