[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/ast.cpp

@@ -117,6 +117,11 @@ void ASTNodeExpressionBase::assign_lvalue_true() {
   this->is_lvalue = true;
 }
 
+void ASTNodeExpressionBase::assign_always_true_or_false(int flow_true_false_state) {
+  this->is_always_true = flow_true_false_state == 1;      // see smart-casts-cfg.h
+  this->is_always_false = flow_true_false_state == 2;
+}
+
 void Vertex<ast_reference>::assign_sym(const Symbol* sym) {
   this->sym = sym;
 }
@@ -173,6 +178,10 @@ void Vertex<ast_is_null_check>::assign_is_negated(bool is_negated) {
   this->is_negated = is_negated;
 }
 
+void Vertex<ast_sequence>::assign_first_unreachable(AnyV first_unreachable) {
+  this->first_unreachable = first_unreachable;
+}
+
 void Vertex<ast_dot_access>::assign_target(const DotTarget& target) {
   this->target = target;
 }