[boost] [hash2] Hash Algorithm Requirements versus concepts

Hi Everyone, It strikes me that the description of Hash Algorithm Requirements does not use C++ concepts. Even though the library is C++11-compatible, it could use a concept in the documentation. The goal of the concepts was to serve a case like this, so I wonder if this is just an omission from the authors, or if this is an indication that concepts do not deliver their promise. There is the optional `block_size` but this could be modeled by two concepts HashAlgorithm and BlockHashAlgorithm. The type of `block_size` looks like a candidate for another concept HashAlgorithmResult. Regards, &rzej;