[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/abscode.cpp

@@ -414,7 +414,7 @@ std::vector<var_idx_t> CodeBlob::create_var(TypePtr var_type, SrcLocation loc, s
     std::string null_flag_name = name.empty() ? name : name + ".NNFlag";
     ir_idx = create_var(t_nullable->inner, loc, std::move(name));
     ir_idx.emplace_back(create_var(TypeDataBool::create(), loc, std::move(null_flag_name))[0]);
-  } else if (var_type != TypeDataVoid::create()) {
+  } else if (var_type != TypeDataVoid::create() && var_type != TypeDataNever::create()) {
 #ifdef TOLK_DEBUG
     tolk_assert(stack_w == 1);
 #endif