intern FFI call names
Instead of storing strings directly in the ops, do as we do for everything else and use the symbol table.
This required duplicating `biscuit_parser::builder::Binary` and `Unary` in the `biscuit_auth::builder` module (which previously used the definitions from the `datalog` module directly). There is a lot of duplication between `biscuit_parser::builder` and `biscuit_auth::builder`, with a circular-ish dependency (biscuit_auth depends on biscuit parser, but code generated by the `ToTokens` impl blocks in biscuit parser depend on `biscuit_auth::builder`).