[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/pipeline.h

@@ -33,6 +33,14 @@ namespace tolk {
 AllSrcFiles pipeline_discover_and_parse_sources(const std::string& stdlib_filename, const std::string& entrypoint_filename);
 
 void pipeline_register_global_symbols(const AllSrcFiles&);
+void pipeline_resolve_identifiers_and_assign_symbols(const AllSrcFiles&);
+void pipeline_calculate_rvalue_lvalue(const AllSrcFiles&);
+void pipeline_detect_unreachable_statements(const AllSrcFiles&);
+void pipeline_infer_and_check_types(const AllSrcFiles&);
+void pipeline_refine_lvalue_for_mutate_arguments(const AllSrcFiles&);
+void pipeline_check_rvalue_lvalue(const AllSrcFiles&);
+void pipeline_check_pure_impure_operations(const AllSrcFiles&);
+void pipeline_constant_folding(const AllSrcFiles&);
 void pipeline_convert_ast_to_legacy_Expr_Op(const AllSrcFiles&);
 
 void pipeline_find_unused_symbols();