Constants should be policy-based

Right now, `boost/sort/spreadsort/detail/constants.hpp` defines a bunch of performance-tuning constants for spreadsort. Why make these constants global? Because these constants live in a simple enum, they're impossible to change without changing the Boost headers, which is awkward when compiling against a Boost installed system-wide.

Instead, spreadsort should accept a struct containing these constants as a template argument. This way, users would be able to customize these parameters for specific invocations of spreadsort without changing the Boost headers themselves.