Clean up parser

[certainty]

This started as a refactoring to clean up the parser and turned into a huge clean-up plus partial rewrite of the frontend.
I'll merge this into main soon and will add another round of clean-up after which refactors some parts that are still too messy. At this point I don't want to continue too far without some clean-up.

Changes

1. The compiler uses denotation tracking to identify special forms and distinguish them from local bindings. In other word things like (define (foo if) (if #t 'yes 'no)) compile correctly and will yield and error, since if is a local binding instead of the if special form in this instance

2. The frontend contains an expansion phase, which is used to expand defines and quasi-quotes for now, but can be extended to expand arbitrary macros (I hope).

3. The representation of Datum and Sexp has been made a bit more ergnomic. Among other things you can display datum and sexp in a convenient way now. Also it's easier to construct Datum now.

4. Support for vector and improper list added. The reader now recognised vectors and improper lists correctly. So now the only missing sexp literal seems to be bytevectors.

5. Error reporting uses codespan_reporting. This improves the error messages for the compiler alot, as it provides nicely formatted information about what went wrong. Well at least in theory. In practice all the machinary is in place, but I didn't give a lot of love to the error messages yet.

6. Various smaller changes in the structure. Elided life-times, moves files and modules, renamed several associated functions.