[Tolk] Compiler built-in __expect_type() for testing purposes

Currently, tolk-tester can test various "output" of the compiler:
pass input and check output, validate fif codegen, etc.
But it can not test compiler internals and AST representation.

I've added an ability to have special functions to check/expose
internal compiler state. The first (and the only now) is:
> __expect_type(some_expr, "<type>");
Such a call has special treatment in a compilation process.
Compilation fails if this expression doesn't have requested type.

It's intended to be used in tests only. Not present in stdlib.

tolk/type-system.h

@@ -417,6 +417,7 @@ public:
 
 class Lexer;
 TypePtr parse_type_from_tokens(Lexer& lex);
+TypePtr parse_type_from_string(std::string_view text);
 
 void type_system_init();