This fixed a compilation error of the OS X 10.11.6 C++ compiler:

    ./fmt/test/format-test.cc:1861:16: error: default initialization of an object of const type 'const Answer' without a user-provided default constructor
      const Answer const_answer;
                   ^
                               {}

* Move has_formatter into the public fmt namespace.

This will allow users to do SFINAE-friendly checks for
the formattability of a type.

Fixes #1369

Found with modernize-deprecated-headers
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with hicpp-use-override
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with hicpp-use-equals-default
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with hicpp-use-auto
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with performance-noexcept-move-constructor
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with bugprone-macro-parentheses
Signed-off-by: Rosen Penev <rosenp@gmail.com>