Be able to fully parse docutils' mathematics.txt

[infinity0]

On a discussion on different latex-mathml converters I found out about the following test file: https://docutils.sourceforge.io/docs/ref/rst/mathematics.txt

Pandoc can parse a lot of it, but gives the following errors:

$ pandoc  --mathml -f rst mathematics.txt 2>&1  >/dev/null  | grep "unexpected" | sort -u
  unexpected "\\"
  unexpected control sequence \arrowvert
  unexpected control sequence \Arrowvert
  unexpected control sequence \Bigl
  unexpected control sequence \bracevert
  unexpected control sequence \cfrac
  unexpected control sequence \circledS
  unexpected control sequence \diagdown
  unexpected control sequence \diagup
  unexpected control sequence \gggtr
  unexpected control sequence \idotsint
  unexpected control sequence \injlim
  unexpected control sequence \intop
  unexpected control sequence \llless
  unexpected control sequence \mspace
  unexpected control sequence \negmedspace
  unexpected control sequence \negthickspace
  unexpected control sequence \ngeqq
  unexpected control sequence \nleqq
  unexpected control sequence \nshortmid
  unexpected control sequence \nshortparallel
  unexpected control sequence \nsubseteqq
  unexpected control sequence \nsupseteqq
  unexpected control sequence \ointop
  unexpected control sequence \projlim
  unexpected control sequence \shortmid
  unexpected control sequence \shortparallel
  unexpected control sequence \smallint
  unexpected control sequence \surd
  unexpected control sequence \thickapprox
  unexpected control sequence \thicksim
  unexpected control sequence \underleftrightarrow
  unexpected control sequence \varinjlim
  unexpected control sequence \varliminf
  unexpected control sequence \varlimsup
  unexpected control sequence \varprojlim
  unexpected "x"

Many of these should just be a case of updating unimathsymbols.txt; some other things are a bit more complex such as spaces \ .