[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/tolk.cpp

@@ -58,8 +58,8 @@ int tolk_proceed(const std::string &entrypoint_filename) {
     pipeline_register_global_symbols();
     pipeline_resolve_identifiers_and_assign_symbols();
     pipeline_calculate_rvalue_lvalue();
-    pipeline_detect_unreachable_statements();
     pipeline_infer_types_and_calls_and_fields();
+    pipeline_check_inferred_types();
     pipeline_refine_lvalue_for_mutate_arguments();
     pipeline_check_rvalue_lvalue();
     pipeline_check_pure_impure_operations();