HaXml: Haskell and XML

## Migrating from HaXml 1.13 to HaXml >= 1.17

There are 3 main changes between 1.13 and 1.17:

### More info fields in the generic document tree

Content items in the generic document representation now have an extra polymorphic component field. This can be used for propagating information throughout the document tree, like namespace scopes. However, if you simply want to change some old code to use the new types, essentially you can just use unit () values as dummies to fill in this field.

`data Document` --> `data Document i`

`data Element` --> `data Element i`

`data Content` --> `data Content i`

### New XmlContent class, replaces old classes

The Haskell2Xml and Xml2Haskell classes have been merged into a single new class: XmlContent. Because the new class is based on combinator parsers that return good error messages, some of the signatures of user-accessible I/O routines have changed to enable the possibility of reporting parse errors. See this page for details. If you are playing with the class methods themselves, then the translation from old-style to new-style is more-or-less like this:

        class XmlContent a where	-- old
          fromElem :: [Content] -> (Maybe a, [Content])
          toElem :: a -> [Content]

becomes

        class XMLContent a where	--new
          parseContents :: XMLParser a
          toContents    :: a -> [Content ()]

### Lazy parsing

Parsing of generic XML documents can now be lazy, that is, it can start returning part of the document tree, before seeing the end of it. However, this does mean that parse errors can be discovered too late to do anything sensible with them - your program will likely just stop with the error message. To use the lazy parser,

`import Text.XML.HaXml.Parse`--> `import Text.XML.HaXml.ParseLazy```