L2U/Constants.pm: Minor fix (phi and varphi).
L2U/Handlers.pm: Add naïve implementation of \overline.
t/simple.t: Add \overline test.
latex2unicode: Add comment block for box functions.
latex2unicode: Fix bug in stackautomaton().
L2U/Handlers.pm: Introduce handle_fooangle().

Along with it: the notion of asymmetric and symmetric delimiters.
handle_rangle() and handle_langle() are now just wrappers around
handle_fooangle().  Lost a bit of code, gained quite a bit of sanity.
t/*: Update for new (consistent) delimiter logic.
L2U/Handlers.pm: Introduce handle_begin(), handle_array() and friends.

One of those rather big foundational steps (with immediate application):
l2u can now deal with latex environments, starting with 'array'.
t/array.t: Add new test category for the 'array' environment.
t/brightmare.t: Enable the first array-test.
L2U/Handlers.pm: Fix asymmetric behaviour of \sum.
t/*: Add regression test for asymmetric \sum; update for new representation.
t/simple.t: Add test for '\hat n' behaviour.
latex2unicode: Fix '\hat n' behaviour; test passes.
L2U/Handlers.pm: Add \mathcal and \mathbb dummy handlers.