Add ImageFormat::from_extension

[a1phyr]

I also moved the inner non-generic function form io::free_functions and removed the intermediate PathError enum.

I license past and future contributions under the dual MIT/Apache-2.0 license,
allowing licensees to chose either at their option.

[a1phyr]

Not-so-related note: I think it would make more sense for ImageFormatHint::PathExtension to contain an OsString instead of a PathBuf, as it stores a mere extension.

This would be a breaking change, however.

[HeroicKatora]

👍 on moving the method and making the error handling more concise. And for homomorphizing the body of the conversion. One small nit on the interface.

[HeroicKatora]

I think it would make more sense for ImageFormatHint::PathExtension to contain an OsString instead of a PathBuf, as it stores a mere extension.

That would indeed be breaking, but adding a new variant shouldn't be.

[HeroicKatora]

Nice, thank you!