GrayA44 issues

[ripytide]

The type alias is inconsistent being GRAYA4 instead of GRAYA44 and the doc-comment is inconsistent:

/// [`Gray<u8>`]
pub type GRAY8 = formats::gray::Gray_v09<u8>;
/// 4 bit for gray and 4 bit for alpha
pub type GRAYA4 = formats::gray_a44::GrayA44;
/// [`GrayA<u8>`]
pub type GRAYA8 = formats::gray_a::GrayA<u8>;

Also should we use a non-named struct parameter GrayA44{v:u8} instead of GrayA44(u8), again consistency. There is the small risk of confusion with GrayA.v but I think that struct pattern consistency is more worth it as that is more likely to be run into by users.

let rgb = Rgb { r: 0, b: 0, g: 0 };
let gray = GrayA44 { v: A8x };