Support for 'user-defined' namespace

[jstrzebonski]

We're using fmt in a library https://github.com/DARMA-tasking/vt, which can be next linked to an app, that itself uses fmt for its own purposes. We'd like to protect ourselves from any kind of ODR violations or ABI mismatch problems, or anything alike. It seems that the simplest way would be to just use our own namespace inside of the fmt namespace.

At first I wanted to simply define FMT_BEGIN_NAMESPACE to something like

#define FMT_BEGIN_NAMESPACE \
  namespace fmt {           \
  FMT_INLINE_NAMESPACE vt {
#endif

but in core.h there's one place where fmt::<version> is explicitly used

void to_string_view(...);
using fmt::v8::to_string_view;

What I did in the end was adding another macro, and replacing explicit references to version, with that macro

#ifndef FMT_INLINE_NAMESPACE_NAME
#  define FMT_INLINE_NAMESPACE_NAME v7
#endif

Could something like this be added to fmt, to add possibility to easily define own namespace for fmt?

For reference, here's a PR to our library with described changes:
DARMA-tasking/vt#1618

[vitaut]

I think v8 is no longer needed in

fmt/include/fmt/core.h

Line 570 in e0136fc

using fmt::v8::to_string_view;

because we require inline namespace support now. A PR to remove it would be welcome.