Allow passing `value_compare` by rvalue_ref.

[kalaxy]

Currently the code only supports making a copy of the comparison object. It would be great to support moving comparison objects into place to permit move-only comparison objects and better efficiency of heavy weight comparison objects.

[kalaxy]

Here is a simple piece of code which demonstrates the desire.

#include <boost/heap/skew_heap.hpp>

struct MoveOnlyCmp {
    MoveOnlyCmp() = default;
    MoveOnlyCmp(MoveOnlyCmp &&) = default;
    MoveOnlyCmp& operator=(MoveOnlyCmp &&) = default;

    // Without these there is a compiler error
    //MoveOnlyCmp(MoveOnlyCmp const&) = default;
    //MoveOnlyCmp& operator=(MoveOnlyCmp const&) = default;

    bool operator()( int lhs, int rhs ) const {
        return lhs < rhs;
    }
};

int main() {
    using PQ = boost::heap::skew_heap<int, boost::heap::compare<MoveOnlyCmp>>;

    MoveOnlyCmp cmp{};

    PQ pq( std::move(cmp) );
    pq.push(1);
    pq.pop();
}