Better output printing.

[DRMacIver]

This introduces draw_named as the primitive for when we are actually printing, which provides much nicer output for values.

It then makes the hegel::test macro automatically rewrite draw() to draw_named where it is safe to do so.

The result is that if you have a test that looks something like:

use hegel::{generators as gs};

#[hegel::test]
fn my_test(tc: hegel::TestCase){
    let mut running_total = 0;

    let iters = tc.draw(gs::integers().min_value(0).max_value(10));

    for _ in 0..iters {
        let summand = tc.draw(gs::integers().min_value(0).max_value(100));
        running_total += summand;
        assert!(running_total <= 100);
    }
}

You will see output like:

let iters = 2;
let summand_1 = 1;
let summand_2 = 100;
thread 'my_test' (2) panicked at examples/new_format.rs:12:9:
assertion failed: running_total <= 100

If you were to write it like this without the intermediate variable:

use hegel::{generators as gs};

#[hegel::test]
fn my_test(tc: hegel::TestCase){
    let mut running_total = 0;

    let iters = tc.draw(gs::integers().min_value(0).max_value(10));

    for _ in 0..iters {
        running_total += tc.draw(gs::integers().min_value(0).max_value(100));
        assert!(running_total <= 100);
    }
}

You would see:

let iters = 2;
let unnamed_1 = 1;
let unnamed_2 = 100;
thread 'my_test' (2) panicked at examples/new_format.rs:11:9:
assertion failed: running_total <= 100

Users are able to name things directly if they want. It's a bit unergonomic but not awful. e.g.

use hegel::{generators as gs};

#[hegel::test]
fn my_test(tc: hegel::TestCase){
    let mut running_total = 0;

    let iters = tc.draw(gs::integers().min_value(0).max_value(10));

    for _ in 0..iters {
        running_total += tc.draw_named(gs::integers().min_value(0).max_value(100), "summand", true);
        assert!(running_total <= 100);
    }
}

is equivalent to the first example.