Update fof parser to accept fof sequents

[SimonGuilloud]

Hi!
Thanks a lot for your parser. For a project of mine, I need to be able to parse sequents (of the form [p, q] --> [r, s, t]), so I extended the parser accordingly. Feel free to merge or not :)
I'm a beginner in Rust, so if there is anything that you don't like in the code, please let me know.

• Update fof parser to accept fof sequents, as in tptp syntax https://tptp.org/UserDocs/TPTPLanguage/SyntaxBNF.html#fof_sequent
• Add tests and update existing tests accordingly.
• Replace assert_display_snapshot by assert_snapshot, since the former is deprecated and causes warnings.