Remove the dependency on std::locale to reduce executable sizes

For embedded target the usage of locale bring a huge amount of code to the executable, just to get hold of the thousands separator. Using a static defined thousands separator makes libfmt embedded target friendly.

Since the commit 691a7a91 compile-time string checking was accidently not possible anymore.
With this change, it should be possible again.

This allows construction of basic_format_args from a dynamic set of arguments. The syntax is a little clunky and could probably be improved but this at least enables the functionality.