Real approx functions

[dragazo]

It's a bit difficult to work with Real (in z3, not z3-sys) since the only provided way to get a value out at the end is Real::as_real(&self) -> Option<(i64, i64)>, which really only works for rational numbers. In this PR I've added two new functions to help:

• Real::approx(&self, precision: usize) -> ::std::string::String, which gives a string approximation of the real value up to precision correct digits after the decimal (via Z3_get_numeral_decimal_string).
• Real::approx_f64(&self) -> f64, which is just a convenience wrapper for self.approx(17).parse().unwrap() to get a full-precision f64 approximation of the real number.

I also noticed that tests/objectives.rs, tests/ops.rs, and tests/semver_tests.rs weren't actually having their tests executed since they weren't linked into the lib, so I declared them as (test) modules. It looks like some of those are failing, but that's not something that this PR caused.

[waywardmonkeys]

I'll fix that clippy error and then you can rebase.

[waywardmonkeys]

Oops, never mind ... that's in your code!

[dragazo]

Sure, I think I just fixed the last of the clippy issues.

[waywardmonkeys]

Thanks!