`0i8.abs() as u8` is not lossy

[jhpratt]

A special case for cast_sign_loss, if the absolute value is taken, the value is guaranteed to be non-negative. As such, a cast from iX to uY where Y >= X is unquestionably safe.

I'm using clippy 0.0.212 (cd3df6b 2019-08-20), though I suspect this hasn't changed in the meantime.

[HMPerson1]

I can work on this