[backend] Drop unused function
[tests] Use proper minus here

Though MathML recommends that hyphen-minus is treated as minus in math
mode, I think this is nonsense and authoring tools should use the proper
Unicode code point of the minus.
[tests] Encode the prime properly

The rendering is not so good, though.
[tests] Use mathvariant, not fontstyle/fontweight
[tests] Use the asterisk operator
[tests] Remove useless test
[backend] Plug some memory leaks
[engine] Plug a memory leak