Schema should support file validation without parsing the document

[flavorjones]

currently Schema#validate, when passed a file, parses the file and passes it to xmlSchemaValidateDoc.

libxml2 provides xmlSchemaValidateFile which allows us to validate a file without parsing a full-blown Nokogiri::XML::Document.

let's use that for files instead.

[flavorjones]

Or actually, probably xmlSchemaValidateStream

[flavorjones]

along with xmlParserInputBufferCreateIO

[flavorjones]

Nokogiri::XML::Schema#validate now uses xmlSchemaValidateFile if a filename is provided. Closed by 20cb6fd