Add conformance tests

[Mingun]

The official test suite is located at https://www.w3.org/XML/Test/. The all links from that page or found by references from that page:

• Links under Releases header. Only last test-suite we should use (this is different versions of the same data)
• direct link to the latest test-suite: https://www.w3.org/XML/Test/xmlts20130923.zip
• CVS repository: https://dev.w3.org/cvsweb/2001/XML-Test-Suite/
• Java generator: https://github.com/belingueres/xml-conformance
• NPM package: https://github.com/lddubeau/xml-conformance-suite

There also exist W3C XML Schema 1.1 (XSD 1.1) test suite, maybe would be useful: https://github.com/w3c/xsdtests

The preferred implementation should automatically download test suite if it does not exist locally and run tests. We also could have a copy as a submodule in our repository. I prefer to use default Rust tools, i.e. cargo test. Test could be implemented in a separated crate to not slow down everyday workflow.

[Mingun]

Some intermediate results of investigation:

1. The official conformance testsuite has very low quality: many repeated tests, weak structure, has errors (file with "pass" in name marked as "invalid", but comment say that it is valid)
2. Found bug in quick-xml (Fix processing instruction parsing #753): we end processing instruction after the first > even if there is no ? before it. Actually, we produce an error in that case. We should continue search of ?> until EOF is reached.
3. Processing instructions:
   • continued to the first ?> which cannot be escaped
   • any parameter entities (%name;) or general entities (&name;) are not recognized
   • character references (&#...;) are not recognized
   • no characters has special meaning. In particular, ' and " does nothing and no need to be paired
   • literal characters restricted in XML still forbidden. So you can write � (which is just string "�"), but cannot use character U+0000