std::advance and std::distance for Boost.Iterator traversal categories

[anonymous]

The new traversal categories of Boost.Iterator should be used in std::advance and std::distance, but this is obviously not possible without the cooperation of compiler writers.

The attached patch (against the trunk, rev. 39675) implements boost::advance and boost::distance that use the new iterator categories. It implements them within the utility library, since that library already contains the closely related next() and prior() function templates. Documentation and test cases are included.

The patch also modifies next() and prior() to use boost::advance(), although this change is a considerably higher risk than the addition of the new feature.

[Dave Abrahams]

Aside from the fact that this is not a patch for the iterator library, the new categories are a dead-end as the standard has lifted the reference requirement on forward iterators, so I don't plan to implement anything like this. I actually plan to deprecate the new categories.