latex2unicode: Rewire parser post- and preprocessing stages.

This is actually a big deal: Now '\in' will not mask '\int' anymore.
In addition, I seem to have paved the way for the "spacious" option.
latex2unicode: Clean up and remove stale find_ext_block().
L2U/Constants.pm: Activate \in (finally); also more relation signs.
L2U/Handlers.pm: Tighten match expressions in handle().