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
