Modernize mspace width values

[dginev]

There is a scary deprecation warning in Firefox:

“veryverythinmathspace”, “verythinmathspace”, “thinmathspace”, “mediummathspace”, “thickmathspace”,
  “verythickmathspace” and “veryverythickmathspace” are 
  deprecated values for MathML lengths and will be removed at a future date.

Changing these in latexml is technically easy -- the question is what should we change them to?

We might as well stay a step ahead of the deprecation status changing.

[fred-wang]

@dginev What's the status of this in LaTeXML?

The replacement proposed is w3c/mathml#75 (comment)

[dginev]

@fred-wang Thank you for pointing out the replacements! And for the issue bump.

I believe @brucemiller and I still intend to get this refactor done for the upcoming release, together with the other spacing-related issue at #1804 .

latexml 0.8.7 is tentatively scheduled for early November, and we'll try to make it as compatible with MathML Core as we can.