The functions and constants under png::chunk should probably be methods and associated constants of png::chunk::ChunkType

[LikeLakers2]

Hi there! I was looking at the png crate, and found everything under the png::chunk module appears to be associated with png::chunk::ChunkType. That is to say, the functions all take a single parameter of type ChunkType, and all the constants are of type ChunkType too.

So I want to suggest that the functions and constants under the png::chunk module be moved to be methods and associated constants of ChunkType. At the least, I don't know of a good reason why they can't be methods and associated constants... If there is a good reason for this, please let me know.

P.S. As an aside, the same issue applies to png::text_metadata::DECOMPRESSION_LIMIT and png::text_metadata::ZTXtChunk.