feat: add run interface

[camshaft]

This change adds the ability to use generator functions inside of harnesses without having to specify the inputs up front.

use bolero::*;

check!().run(|| {
    let _ = any::<u64>();
    let _ = (0..4).any();
    let _ = [1, 2, 3].pick();
    let _ = gen::<Vec<u8>>().with().len(2..4).any();
});

Note that there are a couple of caveats to this interface:

• This uses thread-local variables for each call to any, which can have a bit of overhead, so it's generally preferred to use the .with_type and .with_generator if possible
• The harness does not currently track inputs for displaying when things fail so it may be a bit more difficult to debug. This may change in the future.
• Shrinking is currently not implemented but will be at a later time.