Enhance XML parser/writer/traverser

[wooky]

• Rename __attributes__ and __item__ fields
  • For example, to @attributes@ and @item@
• Improve error reporting
  • If an element is missing, the message Text element __item__ has no text has been previously reported, which is useless
  • In general, provide a wider range of errors, and convert XML errors to Zig ones, rather than returning a generic zupnp.xml.Error
• Support more types
  • float, enum (both numeric and stringy), union, array, pointer (non-string, that's already dealt with)
• Allow parsing without providing an allocator
  • With some exceptions, e.g. parsing into any slice that's not a string requires an allocator
• Verify that the target struct doesn't have both @item@ and non-@attributes@ fields
• Support parsing XML documents that do not have one root node
  • Currently the library crashes if a document like <tag1 /><tag2 /> is provided
• Use default values if the parser doesn't find a tag in the document
• Differentiate between missing tags and empty strings inside the parser