[build] Remove unused files
[build] Cleanup the scripts directory
[engine] Make “largeop” synonym to “stretchy”

Not sure if we should treat large operators like TeX does (i.e. only
having two sizes, display and text), but this should restore the old
behaviour for now.

Fixes #17.
[engine] Simplify a bit
[backend] Fix stretching in both directions

If we are stretching an operator in both direction, the horizontal
stretching ends up discarding the result of the vertical stretching that
preceded it.

This fixes #16 as a side effect, though I think there is something else
wrong there, as we should’t be stretching horizontally to begin with.
[engine] Avoid requesting stretch needlessly

Don’t try to fix the value of the stretch if no stretch is requested,
otherwise we might end up trying to horizontally stretch stretchy
operators that has non-zero lspace or rspace because of the difference
between the natural width and the spaced one.

Related to #16.