This removes the use of `enable_if` and tries to adopt concepts instead
of type traits when available.
The `ostream.h` change is a bit subtle as it adds a restriction not
previously in place -- that the stream is *contvertible* to
`std::ostream` as well as having it as a base class. This seems to match
the intent of the code.
The `hashing.h` code adds an implementation detail concept, and so I've
also clarified that the dispatch namespace is an internal one that isn't
part of the public API.
Chandler Carruth
2e236759ca