fmt::ptr: Support function pointers

[mikecrowe]

Passing a function pointer to fmt::ptr results in:

In file included from /home/mac/git/fmt/test/gmock/gmock.h:238,
from /home/mac/git/fmt/test/format-test.cc:31:
.../fmt/test/format-test.cc: In member function ‘virtual void FormatterTest_FormatPointer_Test::TestBody()’:
.../fmt/test/format-test.cc:1486:56: error: no matching function for call to ‘ptr(void (&)(int, double, std::__cxx11::string))’
format("{}", fmt::ptr(function_pointer_test)));

Let's add an overload to support that usage.

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[mikecrowe]

Using detail::bit_cast rather than reinterpret_cast appears to have been sufficient to make g++4.8 happy. However, I'm not sure what planet MSVC is on with:

D:\a\fmt\fmt\test\format-test.cc(1485,1): error C2668: 'fmt::v7::ptr': ambiguous call to overloaded function [D:\a\fmt\build\test\format-test.vcxproj]
D:\a\fmt\fmt\include\fmt/format.h(3742,60): message : could be 'const void *fmt::v7::ptr<void,int,double,std::string>(T (__cdecl *)(int,double,std::string))' [D:\a\fmt\build\test\format-test.vcxproj]
          with
          [
              T=void
          ]
D:\a\fmt\fmt\include\fmt/format.h(3735,42): message : or       'const void *fmt::v7::ptr<void(int,double,std::string)>(T (__cdecl *))' [D:\a\fmt\build\test\format-test.vcxproj]
          with
          [
              T=void (int,double,std::string)
          ]
D:\a\fmt\fmt\test\format-test.cc(1486,1): message : while trying to match the argument list '(overloaded-function)' [D:\a\fmt\build\test\format-test.vcxproj]

I suppose this means that the overload isn't even necessary for MSVC.

[vitaut]

Thank you!