Add an explicit specialization of container_hash::is_described_class …

[pdimov]

…for filesystem::path, because when reflection is available, path becomes a described class and the hash_value call becomes ambiguous

[Lastique]

is_described_class isn't declared anywhere in Boost.Filesystem, and we don't include Boost.ContainerHash.

But I wonder if this should be necessary. Doesn't this PR indicate potential breakages in the users' code? Can the generic hash_value overload be made less preferable in overload resolution? For example, mark the argument as const volatile reference?

[pdimov]

It's declared in hash_fwd.hpp that's already being included, which is why I didn't include the declaration.

Yes, it does indicate potential breakage in user code. No, I won't add volatile to the reference. :-)

Specializing the ContainerHash classification traits is already the documented method of resolving such ambiguities. That's not a very common problem because user overloads are typically more specialized.