Add lens support eg. with xeno-lens

[AlistairB]

Hi,

I currently use hexml primarily because there is hexml-lens which simplifies finding the data I need to pull out the XML quite a lot. hexml (and hexml-lens) is fairly unmaintained and it has some other problems for me. Would something like xeno-lens fit in with this project?

Thanks

[ocramz]

@AlistairB I think a xeno-lens would be a great addition! I'd suggesting basing it on microlens.