Decouple HaRP transformation from XML transformation.

[niklasbroberg]

There is a clear dependency between regular patterns, where XmlSyntax transforms into RegularPatterns which in turn transforms into vanilla Haskell. It would simplify the code greatly (in particular that for transforming away XmlSyntax in expressions) if these transformations were done in two separate passes.

Note that XmlSyntax only depends on RegularPatterns in practice if pattern matching over XML is used. For real-world uses of XmlSyntax, this is only very rarely the case. We could thus consider a more fine-grained model, where XmlSyntax enables XML expressions, and a new extension XmlPatterns (which depends on/implies both RegularPatterns and XmlSyntax) handles the pattern side.