Release Support serve >=1.0.181, various serde fixes and async writer · tafia/quick-xml

What's Changed

MSRV bumped to 1.56! Crate now uses Rust 2021 edition.

⚠️ BREAKING CHANGE! Serde's enum representation was changed (it was buggy anyway) to ensure compatibility with serde >= 1.0.181.

#545: Resolve well-known namespaces (xml and xmlns) to their appropriate URIs. Also, enforce namespace constraints related to these well-known namespaces.
#635: Add support for async ElementWriter operations.

#660: Fixed incorrect deserialization of xs:lists from empty tags (<tag/> or <tag></tag>). Previously an DeError::UnexpectedEof") was returned in that case.
#580: Fixed incorrect deserialization of vectors of newtypes from sequences of tags.
#661: More string handling of serialized primitive values (booleans, numbers, strings, unit structs, unit variants). <int>123<something-else/></int> is no longer valid content. Previously all data after 123 up to closing tag would be silently skipped.
#567: Fixed incorrect deserialization of vectors of enums from sequences of tags.
#671: Fixed deserialization of empty simpleTypes (for example, attributes) into Option fields: now they are always deserialized as Some("").

#643: Bumped MSRV to 1.56. In practice the previous MSRV was incorrect in many cases.
#643: Adopted Rust 2021 edition.
#545: Added new Error variant -- Error::InvalidPrefixBind.
#651: Relax requirement for version of arbitrary dependency -- we're actually compatible with version 1.0.0 and up.
#649: Make features linkable and reference them in the docs.
#619: Allow to raise application errors in ElementWriter::write_inner_content (and newly added ElementWriter::write_inner_content_async of course).
#662: Get rid of some allocations during serde deserialization.
#665: Improve serialization of xs:lists when some elements serialized to an empty string.
#630: Fixed compatibility with serde >= 1.0.181

Full Changelog: v0.30.0...v0.31.0