implement feature request #24

Everything should be there, some of it under a different name:

* ``aux.font_has_glyph()``, non ``check_char()``
* ``aux.provides_script()``, non ``check_script()``
* ``aux.provides_language()``, non ``check_language()``
* ``aux.provides_feature()``, non  ``check_feature()``
* ``aux.get_math_dimension()`` and ``aux.sprint_math_dimension()``, non ``get_mathfontdimen()``
* ``aux.name_of_codepoint()`` and ``aux.codepoint_of_name()``, non ``name_to_slot()``

The ``provides_*`` stuff also have their arguments reordered. The functionality is the same, however, and it shouldn’t be a problem to write fallbacks for the original names.

There are four new tests that demonstrate their usage, see ``tests/pln-aux-*tex``.