Rule model should allow evaluating constant expressions

[aannleax]

Nemo rule api should include a function to evaluate constant expressions:

let term = Term::parse("1 + 2");
let result = term.evaluate(); // TODO

println!("{}", result) // 3

The evaluation should use the same pipeline as during reasoning to avoid mismatches.

Building on this, it should also be possible to evaluate non-ground terms by providing a grounding:

let term = Term::parse("?x + 2");
let grounding = Grounding::from(vec![(Term::parse("?x"), Term::parse("1")]); // TODO
let result = term.evaluate_grounded(grouding); // TODO

println!("{}", result) // 3