oxml: extract nsmap to docx.oxml.ns
oxml: extract parser to docx.oxml

register_custom_element_class() needs to go with to avoid a circular
import.
oxml: extract NamespacePrefixedTag to docx.oxml.ns

And added unit tests for NamespacePrefixedTag
oxml: extract nsdecls() to docx.oxml.ns
oxml: extract nspfxmap() to docx.oxml.ns

Changes to OxmlElement were just to satisfy flake8, although it’s
probably a little bit better style anyway.
oxml: extract qn() to docx.oxml.ns
oxml: extract parse_xml() to pptx.oxml.__init__

Was `oxml_fromstring()`. Also renamed separate function of same name in
pptx.opc.oxml having non-intersecting scope.
oxml: remove dead _SubElement()
oxml: extract OxmlElement() to pptx.oxml
oxml: extract XmlString to oxml.xmlchemy

serialize_for_reading() had to go at the same time to avoid a circular
import dependency.
oxml: add unit tests for serialize_for_reading()
oxml: add unit tests for oxml_parser
oxml: add unit tests for parse_xml()
oxml: add unit tests for register_element_cls()

renamed from register_custom_element_class()
oxml: add unit tests for OxmlElement
oxml: mv BaseOxmlElement to docx.oxml.xmlchemy

was OxmlBaseElement
oxml: add unit tests for BaseOxmlElement
xmlch: add BaseOxmlElement.remove_all()
xmlch: add BaseOxmlElement.xpath()
xmlch: transplant xmlchemy meta classes

Entailed basic simple types.