Introduce an intermediate representation of the parsed tree

[Kineolyan]

This would allow for multiple parsers to produce the same IR, not to have to reimplement the whole bytecode generation.
This would also allow for a Rust backend, though as the parser is currently in Rust, we could simply skip the bytecode generation and play with the existing representation.
This is not an important change, as it is more interesting to simulate an actual computer instead of adding parsers in other languages.