Avoid namespace clash for fmt

## Problem

In the case of an existing `fmt` namespace (in my project this looks like `Project::fmt`) it is possible to get a namespace clash in debug builds (MSVC 2017)

## Proposed Solution

When referencing `fmt` internally, be explicit that it is relative to the global namespace using `::fmt`

include/fmt/core.h

@@ -253,7 +253,7 @@ FMT_API void assert_fail(const char* file, int line, const char* message);
 #    define FMT_ASSERT(condition, message) \
       ((condition)                         \
            ? void()                        \
-           : fmt::internal::assert_fail(__FILE__, __LINE__, (message)))
+           : ::fmt::internal::assert_fail(__FILE__, __LINE__, (message)))
 #  endif
 #endif