parse ¬ (\neg) as a prefix operator

[stevengj]

As discussed on julia-users, this patch parses ¬ as a prefix operator like !, although it doesn't define it by default (since ! is already perfectly good).

The main arguments for this, in my mind, are:

• There is nothing sensible to do with ¬ except to parse it as a prefix operator.
• We already parse the other logic symbols (e.g. ∧) as operators, so we might as well complete the set.

@jverzani also has a use-case in SymPy.

[StefanKarpinski]

+1