is_contiguous_iterator should be a public customization point

Currently the library specializes is_contiguous_iterator for containers it knows to have contiguous iterators. However, it seems likely that users will have their own vector-like containers (indeed, there are several in Boost already). It would be helpful if is_contiguous_iterator was a documented customization point by which users could mark their own iterators as contiguous.

(Indeed, the library already misses one standard one: std::string::iterator)
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