improve positioning of sub- and superscripts

[manuelbb-upb]

this pull should help with #93:

and #105:

the changes are a bit involved;

• we actually try to load the MATH table of the :math font to get recommended constants for typesetting y-positions
• rules similar to the TeXBook PDF are applied; it's a bit complicated to follow, unfortunately
• primes get special treatment:
  • as before L"f'" will be treated as a superscript; but other unicode symbols work, too (this is what unicode-math does, I think)
  • doube and tripple ticks are converted to appropriate unicode symbols ″ and ‷
  • however, \prime gives just the glyph
  • i.e., the string L"x' f' x'' f'' x′ f′ x\prime f\prime x^\prime f^\prime x'_y f_g'" gives
• some more prime commands are defined (\dprime, \backdprime,...)
• scaling is now constant from decoration level 2 upwards; L"A^{B^{C^{D^E}}}_{F_{G_{H_I}}} f^{A'}" gives

The level dependent styling required changes to the state structure; this feels a bit hacky, but easier than inspecting the stack.

Here is the reference images from master:

and the current pull:

[manuelbb-upb]

If OpenType Table parsing is too adventurous, constant defaults can be used.
Otherwise we can go all in and even check for the ssty feature to get nice sub- and superscript glyphs, see the ssty branch.

[manuelbb-upb]

I have marked this "draft" again so that we can decide on whether or not to simply use constants or try to parse opentype tables.
I think the latter option is nicer in that results should look closer to LuaLaTeX output, but I am always a bit nervous around ccall. Locally, things seem to work fine. Should we go for table parsing, I will merge the ssty branch into this pull request, so that we can even have dedicated scriptstyle symbols.