Schema support and document serialization

[0.2.13] 2020-16-01

Thanks to @jangernert for the upgrades to Document serialization.
Thanks to @lweberk for contributing the Schema featureset and to @cbarber for refining the FFI interop.

Added

• Document::to_string_with_options allowing to customize document serialization
• Document::SaveOptions containing the currently supported serialization options, as provided internally by libxml
• Schema holding and managing xmlSchemaPtr as created while parsing by SchemaParserContext
• SchemaParserContext holding source of XSD and parsing into a Schema while gathering and –in case returning– errors that arise from the XSD parser across the FFI to libxml
• SchemaValidationContext holding the Schema from resulting SchemaParserContext parse and offering validation methods for Document, Node or file path to XML, while gathering and –in case returning– validation errors from the XML validator across the FFI to libxml

Changed

• the Document::to_string() serialization method is now implemented through fmt::Display and no longer takes an optional boolean flag. The default behavior is now unformatted serialization - previously to_string(false), while to_string(true) can be realized via

  .to_string_with_options(SaveOptions { format: true, ..SaveOptions::default()})`

See #58 and #64 for implementation details.