Only build debug representations of arguments on failing tests

[davidrusu]

fixes #303

This cuts my prop test run-times nearly in half!

Here's the new flamegraph showing nearly 70% of time spent in test, previously this was 40%.

compare to prior run:

[davidrusu]

@BurntSushi this change nets us a 50% reduction in quickcheck's overhead

It's a fairly small change, so hopefully quick to review. I'd love to see it merged in soon.

[neithernut]

There are bug-fixes pending for months. I wouldn't get my hopes up.

[dan-da]

+1 for getting this merged asap. else I'll need to start referencing @davidrusu's fork in our crates instead.

[niyaznigmatullin]

+1 is there anyone who wants to maintain this crate, probably we ask @BurntSushi to give rights or just use some other fork, @davidrusu's for instance, to continue developing this. The same is with crates.io, we could just create another crate