Migration to antlr4

[rkubosz]

I have no access to antlr3 bindings for python on my distro, so I thought maybe the project could be migrated to antlr4.

Here are some easy changes, that were suggested by antlr4 error log. There is still some grammar rule, that should be updated, but since I have no experience with antlr I left it as it is. I will be very happy if you could look at it and suggest what to do next.

[Zannick]

Hey, thanks for your interest, and sorry for the trouble. You should be able to install the required antlr3 files by downloading them from the antlr3 website or from the github. See the INSTALL file in this repo for more details.

As for antlr4, I did at one point work on changing it in a branch I never pushed to github (...in 2015, oh dear) but unfortunately I haven't had the time to complete it to see how it would work, and I don't want to incorporate it into main until I'm happy with it. The major difference (as I understand it) is that all of the annotations and rewrite syntax used to modify the result AST are gone (so the ^ and the -> and everything after it). (I was also using .g4 files for syntax highlighting purposes and to keep both versions side-by-side, but that's relatively minor.)