Scryer-prolog: Proposition: Bring back prolog_parser

[notoria]

scryer-prolog is sometimes out of sync with prolog_parser and trying to use an old version of scryer-prolog with cargo install scryer-prolog --version 0.8.121 or the published one that is a bit old created issue like #516 .

By having both in the same crate, only a path is required to be specified, there wouldn't be this issue in the future and num-rug-adapter could also be part of the crate and have more attention.

[triska]

It would also simplify the discussion and following issues: The parser attracts only a few, yet very interesting issues, so it would be worthwhile having them appear in the main repository for more publicity.

[XVilka]

It's possible to just put multiple crates into the same repository with the Cargo workspaces: https://doc.rust-lang.org/book/ch14-03-cargo-workspaces.html