add TryFrom<usize> impl for Level and LevelFilter

[nebkor]

This exposes the private Level::from_usize() and LevelFilter::from_usize() functions as fallible conversions. My motivation is to avoid having to use std::mem::transmute(), which is unsafe, when I want to convert a value I got doing my_log_level as usize back to a LevelFilter.

[nebkor]

My specific case was to use it in this function:

fn change_log_level(delta: isize) {
    let level = log::max_level() as isize;
    let max = log::LevelFilter::max() as isize;
    let level = (level + delta).clamp(0, max) as usize;
    // levelfilter is repr(usize) so this is safe
    let level = unsafe { std::mem::transmute::<usize, LevelFilter>(level) };

    log::set_max_level(level);
}

and I was surprised I had to reach for transmute (or otherwise do something clunky like match on each level).

[Thomasdezeeuw]

I don't think we want to commit to this, see #460.

[nebkor]

I don't think we want to commit to this, see #460.

I see that issue except as was pointed out there, and if I may point it out here, you're already committed to repr(usize).