The math tree, LaTeX, linear-format, symbol and tree-walk modules come
from craft-wordmodel (storytold/craft-libs#963, WordCraft's crates/doc at
07f97737) unchanged, with the run colour (Rgb) and the symbol-font tables
they use. MathML is WordCraft's crates/formats/src/mathml.rs and OMML is
WordCraft's crates/docx/src/read/math.rs and write/math.rs (06c2ae6f /
07f97737), moved onto craft-xml. The HTML-specific MathML tests stay in
WordCraft; the rest moved with the code.
mathml::parse and omml::parse/read_str/write_str are new: string entry
points for callers that have no tree of their own.
