Add rules to map from combining to non-combining accents

Created by: fred-wang

cc @khaledhosny @davidcarlisle

I think the implementation note should have an explicit rule to map from combining to non-combining accents. Rationale: MathML suggests to use non-combining characters for operators and most of the operators in its dictionary are non-combining ; however TeX uses combining characters and so does OpenType math fonts.

See https://bugs.webkit.org/show_bug.cgi?id=159513 and https://bugzilla.mozilla.org/show_bug.cgi?id=1285277

TODO: also report this to the Math WG mailing list, if not already done.