Using floating point numbers is problematic because comparison is inexact.
They also take more space than integral numbers in this case.
Add `FromPrimitve`, `ToPrimitive` and `Default` traits to quality newtype.
Closes: #330
BREAKING_CHANGE: Replace f32 quality values in quality items with a
Quality(u16) newtype. Valid values are from 0 to 1000.
- Some stray deriving -> derive changes
- use::{mod} -> use::{self}
- fmt.write -> fmt.write_str
This does not catch the last case of fmt.write_str in the
Show impl of a Header Item. This will need to be changed
separately.
