Allows the creation of a queue using C-like
comparison values. That is to say, cmp
should return a value less than, equal to,
or greater than 0 when l should be placed
before, is equal to, or be placed after r
respectively.
A priority queue implemented using a red black
tree. The ordering supplied must satisfy the assymetry
and transitivity rules as outlined by the dorumentation
of std::cmp::PartialOrd.