[Tolk] AST-based semantic analysis, get rid of Expr

This is a huge refactoring focusing on untangling compiler internals
(previously forked from FunC).
The goal is to convert AST directly to Op (a kind of IR representation),
doing all code analysis at AST level.

Noteable changes:
- AST-based semantic kernel includes: registering global symbols,
  scope handling and resolving local/global identifiers,
  lvalue/rvalue calc and check, implicit return detection,
  mutability analysis, pure/impure validity checks,
  simple constant folding
- values of `const` variables are calculated NOT based on CodeBlob,
  but via a newly-introduced AST-based constant evaluator
- AST vertices are now inherited from expression/statement/other;
  expression vertices have common properties (TypeExpr, lvalue/rvalue)
- symbol table is rewritten completely, SymDef/SymVal no longer exist,
  lexer now doesn't need to register identifiers
- AST vertices have references to symbols, filled at different
  stages of pipeline
- the remaining "FunC legacy part" is almost unchanged besides Expr
  which was fully dropped; AST is converted to Ops (IR) directly

tolk/src-file.h

@@ -18,11 +18,10 @@
 
 #include <string>
 #include <vector>
+#include "fwd-declarations.h"
 
 namespace tolk {
 
-struct ASTNodeBase;
-
 struct SrcFile {
   struct SrcPosition {
     int offset;
@@ -39,7 +38,7 @@ struct SrcFile {
   std::string rel_filename;             // relative to cwd
   std::string abs_filename;             // absolute from root
   std::string text;                     // file contents loaded into memory, every Token::str_val points inside it
-  const ASTNodeBase* ast = nullptr;     // when a file has been parsed, its ast_tolk_file is kept here
+  AnyV ast = nullptr;                   // when a file has been parsed, its ast_tolk_file is kept here
   std::vector<ImportStatement> imports; // to check strictness (can't use a symbol without importing its file)
 
   SrcFile(int file_id, std::string rel_filename, std::string abs_filename, std::string&& text)