Update XML to match convention

[altavir]

stringify -> encodeToString
toXML -> encodeToXml
parse -> decodeFromXml

Right now you have both, but it is a bit confusing. I think it won't hurt to use canonical names.

[pdvrieze]

These are on the list. I figured I'd keep the old ones around a while to allow people more time to move. I can make the deprecations errors though.

[pdvrieze]

I've reviewed these functions. All of them should be properly deprecated. At the same time I have moved the error level to ERROR for some that are quite old now. I will keep a lookout for the warning level deprecations and move them to error next major release (and remove them after that).