Fix fallback to runtime API from compile-time API (#2143)

* fix fallback to the runtime API, add FMT_ENABLE_FALLBACK_TO_RUNTIME_API define, add test

* remove `FMT_ENABLE_FALLBACK_TO_RUNTIME_API`

* pass format string to format_to() inside format_to_n() in compile-time API

instead of compiling it inside format_to_n(), to eliminate code duplication
2 files changed