Make Decoder/Encoded non-generic

[RazrFalcon]

Right now, if I want to decode/encode a PNG from a slice and from a file, I have to instance the whole png crate twice. Which blow ups the binary size.

It would be better and less error-prone to have a non-generic implementation. The public API can still be generic, in theory. But the actual decoding/encoding code should be instanced only once.

[HeroicKatora]

Have you tried using dyn BufRead instead? The performance difference should be negligible, the reason why png can't use it is more or less due to auto-trait (Send + Sync) and lifetime accuracy.

[RazrFalcon]

Right now, I'm simply reading the file into memory and then decode it as slice.