Implement ast::Float::as_f64

[waywardmonkeys]

Fixes Z3Prover/z3#6958

[waywardmonkeys]

@Pat-Lafon Any thoughts on this?

[Pat-Lafon]

Simple enough! Though it is kinda weird that the other .as_*() functions return an optional. Here sometimes you will get 0.0 because the error facilities are suppressed.

#[test]
fn test_float() {
    let cfg = Config::new();
    let ctx = Context::new(&cfg);

/*     let f = ast::Float::from_f64(&ctx, 1.0); */
    let f = ast::Float::new_const_double(&ctx, "test");
    assert_eq!(f.as_f64(), 0.0);
}

[waywardmonkeys]

The underlying API from Z3 differs though which is kind of unfortunate. The C++ binding checks the error code. The C# binding does not. Interestingly (perhaps), other bindings don't seem to call Z3_get_numeral_double().

Maybe I should do so here and return an Option? Although it would be nice to have a more ergonomic way to check that error than we do now ... (There's so much un-ergonomic about this code.)

[Pat-Lafon]

What do you think would be the more ergonomic solution? Should most of these functions return a result type so the error is not suppressed? (Relying on a lot of ? everywhere) Runtime assertion panic on a case by case basis?