[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/src-file.h

@@ -124,10 +124,14 @@ struct Fatal final : std::exception {
 std::ostream& operator<<(std::ostream& os, const Fatal& fatal);
 
 struct ParseError : std::exception {
-  SrcLocation where;
+  FunctionPtr current_function;
+  SrcLocation loc;
   std::string message;
-  ParseError(SrcLocation _where, std::string _msg) : where(_where), message(std::move(_msg)) {
-  }
+
+  ParseError(SrcLocation loc, std::string message)
+    : current_function(nullptr), loc(loc), message(std::move(message)) {}
+  ParseError(FunctionPtr current_function, SrcLocation loc, std::string message)
+    : current_function(current_function), loc(loc), message(std::move(message)) {}
 
   const char* what() const noexcept override {
     return message.c_str();