extend _force_font attribute to be either 'true' or 'color', the latter recording the current color (if different from context), this allows recording the color needed for empty elements that end up generating display (eg mathml)
make hints not disappear from unparsed math; carry over color into mfrac, msqrt and mroot
let stretchy attribute be used when DefMath uses Rewrite
remove xEquals; default to stretchy=>false for delimiters; preserve color on several constructs (root, frac, etc)
preserve color on several forms; add \@defaultunits; use XEquals
use new _force_font format (true or color); preserve color on some constructs
updated tests for default non-stretchiness