[Tolk] Embedded stdlib.tolk, CompilerState, strict includes

Several related changes:
- stdlib.tolk is embedded into a distribution (deb package or tolk-js),
  the user won't have to download it and store as a project file;
  it's an important step to maintain correct language versioning
- stdlib.tolk is auto-included, that's why all its functions are
  available out of the box
- strict includes: you can't use symbol `f` from another file
  unless you've #include'd this file
- drop all C++ global variables holding compilation state,
  merge them into a single struct CompilerState located at
  compiler-state.h; for instance, stdlib filename is also there

tolk/lexer.h

@@ -208,6 +208,7 @@ public:
   std::string_view cur_str() const { return cur_token.str_val; }
   std::string cur_str_std_string() const { return static_cast<std::string>(cur_token.str_val); }
   SrcLocation cur_location() const { return location; }
+  const SrcFile* cur_file() const { return file; }
   int cur_sym_idx() const;
 
   void next();