testutil: generalize testutil.dom.equiv to handle text nodes.
add builders for SVG text elements (<text>, <title>, <desc>).
Add builder for SVG <use>.
rename Defs => defs and have it inherit from Element, not Builder.
add tests for svg.transform.
move id and class setters into svg.Element, since they apply to all elements.
improve test coverage for <svg> and <rect> and hook up new test classes.
remove class and id setters from subclasses, now that they are in svg.Element.
add pre-commit script.