Boost.Fusion extension for std::array

An attempt to adapt std::array for Boost.Fusion.

The implementation is functional and passes the same test as adapted Boost.Array.

However, there is a problem with ADL and range-based access to the sequence which is a well-known issue:

• Fusion begin/end ADL issues with C++0x range-based for
• Boost ticket: [https://svn.boost.org/trac/boost/ticket/4028 fusion::begin and fusion::end lead to problems with ADL-based begin and end functions for ranges]
• Boost ticket: [https://svn.boost.org/trac/boost/ticket/8241 Treat std::array as a Fusion sequence]