test: extract unitutil.mock and unitutil.file
test: transplant tests.unitutil.cxml
test: refactor test_section to use cxml
test: refactor oxml.parts.test_document to cxml
test: refactor oxml.test_text.py to use cxml
test: refactor parts.test_document.py to use cxml
test: refactor tests.test_text to use cxml
test: refactor tests.test_section to use cxml
test: refactor tests.test_shape to use cxml
test: refactor tests.test_table to use cxml
opc: introduce XmlPart subclass

Takes care of all the common responsibilities of XML-content parts,
like parsing and reserializing the XML.
doc: base _Body on Parented

Improved some names along the way, document_part instead of document
where an instance of DocumentPart.
tbl: base Table on Parented
tbl: base _Columns on Parented
tbl: base _Rows on Parented
tbl: base _Column on Parented
tbl: base _Row on Parented
tbl: base _ColumnCells on Parented
tbl: base _RowCells on Parented
tbl: base _Cell on Parented