make namespace explicit when accessing fmt from fmt::internal

[svigerske]

When compiling with MSVC and /permissive-, we had problems in format.cc.
Members of the anonymous namespace inside fmt were not found when trying to use them from fmt::internal.
With the changes from this PR, we avoid this problem.

[svigerske]

As a side note, there are other issues that prevent compilation with /permissive-. Apparently Microsofts own header files are not ready for this so far: https://docs.microsoft.com/en-us/cpp/build/reference/permissive-standards-conformance?view=msvc-170#windows-header-issues

Adding a

struct IUnknown;

before any

#include <windows.h>

works around these.
As this is not a problem with ampl::mp, I didn't include this here.

[glebbelov]

Merged. The workaround for <windows.h> can be added if necessary.