libfmt issue for cpp

[wg485733]

https://github.com/chronoxor/FastBinaryEncoding/commit/5d98a626c6cc3ce9483dbe1c56551f0e60e3792b

2 issues

1. in this commit, the macro only checks for FMT_VERSION. but ostream_formatter is only introduced after fmt 9.0.0. So the generated code would fail with earlier version of fmt.

2. think the user needs to include fmt/ostream.h header file for this to work? while FMT_VERSION is defined in fmt/core.h, so it's worth mentioning that user should include fmt/ostream.h if she already includes fmt/core.h