Serde helpers and indentation for async writer

What's Changed

New Features

• #601: Add serde_helper module to the crate root with some useful utility
  functions and document using of enum's unit variants as a text content of element.
• #606: Implement indentation for AsyncWrite trait implementations.

Bug Fixes

• #603: Fix a regression from #581 that an XML comment or a processing
  instruction between a <!DOCTYPE> and the root element in the file brokes
  deserialization of structs by returning DeError::ExpectedStart
• #608: Return a new error Error::EmptyDocType on empty doctype instead
  of crashing because of a debug assertion.

Misc Changes

• #594: Add a helper macro to help deserialize internally tagged enums
  with Serde, which doesn't work out-of-box due to serde limitations.

New Contributors

• @damb made their first contribution in #606
• @danjpgriffin made their first contribution in #603
• @Kriskras99 made their first contribution in #594

Full Changelog: v0.28.2...v0.29.0