Maybe use --release by default

[sourcefrog]

cargo mutants does many incremental builds and test runs. In most trees, most of the time is spent running the tests.

Tests will typically run faster with --release; in some crates it will be much faster.

So that should at least be an option (#59) but according to the principle of "make it easy" perhaps --release should be the default.

There are at least two potential downsides:

1. Conceivably in some trees the performance hit from doing the initial and incremental builds in release mode will outweigh the benefit of the tests being faster. However, even if that is true sometimes it's probably a win across most trees.
2. Some projects might have test suites that are less rigorous, or simply don't work properly, in release mode. For example they might rely on debug_assert to catch some bugs. I think this would be an unidiomatic use of Rust, but it might well happen.

I think those could both reasonably be handled by having an option to go back to debug builds.

[sourcefrog]

It will probably help on some crates, but I'm not sure it's a good default. Some crates like time take a long time to compile and link with --release, and have relatively quick test suites. Probably better to leave this to the user.