StructTypes encourage piracy

Suppose I have a package, in which I need to serialize some data that contain a LongDNA{4}, but do not want to expose the private (non-documented) memory layout of LongDNA. In that case, I'd need to overwrite StructTypes.StructType(::Type{LongDNA{4}}) - or, alternatively, overwrite some JSON3 methods.

Both are type piracy, which can have rather bad consequences. For example, it's unknown to me if the user installs another package which also need to serialize LongDNA, but may choose to do it differently. In that case, my code may randomly malfunction.

I don't see a non-breaking way of getting around it, but perhaps it's worth considering for a potential breaking release in the future:

• This package defines a struct AbstractEncoder end
• Struct mapping is done by StructTypes.StructType(::AbstractEncoder, ::Type{MyType})
• Default struct mapping can use ::AbstractEncoder directly.
• Users can then subtype AbstractEncoder, and create struct mapping using their own subtype.

Review the StructTypes.StructType methods and the JSON3 methods mentioned in the issue, then compare them with the proposed AbstractEncoder design. Determine whether encoder-specific struct mappings could avoid type piracy without breaking existing users; done means producing an agreed design for a future breaking release.