t/*: Add three simple, surprising tests (integral.t and pathologic.t).
latex2unicode: Expand normalize_input(), noop densify().

Protect stand-alone commands, eg. turn '\a b' into '\a{} b' and deal
with math-mode spacing macros ('\;' and friends).  Also, it turns out
that I now don't need densify() to pass my tests (which I now do).
L2U/Constants.pm: Add math-mode spacing macros.