[Tolk] Compilation pipeline, register global symbols in advance

Since I've implemented AST, now I can drop forward declarations.
Instead, I traverse AST of all files and register global symbols
(functions, constants, global vars) as a separate step, in advance.

That's why, while converting AST to Expr/Op, all available symbols are
already registered.
This greatly simplifies "intermediate state" of yet unknown functions
and checking them afterward.

Redeclaration of local variables (inside the same scope)
is now also prohibited.

tolk/lexer.cpp

@@ -629,7 +629,7 @@ void lexer_init() {
 // Hence, it's difficult to measure Lexer performance separately.
 // This function can be called just to tick Lexer performance, it just scans all input files.
 // There is no sense to use it in production, but when refactoring and optimizing Lexer, it's useful.
-void lexer_measure_performance(const std::vector<SrcFile*>& files_to_just_parse) {
+void lexer_measure_performance(const AllSrcFiles& files_to_just_parse) {
   for (const SrcFile* file : files_to_just_parse) {
     Lexer lex(file);
     while (!lex.is_eof()) {