Fix some warnings

[peterbell10]

This fixes a few warnings I was seeing compiling with clang:

• -Wsigned-enum-bitfield from using standard enums in a bitfield, fixed by giving them an unsigned underlying type.
• -Wundefined-func-template from templates without definitions, fixed by adding extern template declarations.
• -Wsign-conversion from constructing a string_view with an int size, fixed with internal::to_unsigned.

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

[vitaut]

Thanks for the PR. Please move all the extern template declarations to

fmt/include/fmt/format.h

Lines 3337 to 3340 in 59fe455

	#ifndef FMT_HEADER_ONLY
	extern template format_context::iterator internal::vformat_to(
	internal::buffer<char>&, string_view, basic_format_args<format_context>);
	#endif

to avoid introducing extra #ifdef blocks.

[vitaut]

Thanks!

[vitaut]

Reverted because of

include/fmt/format.h:1039:19: error: ‘fmt::v6::basic_format_specs<char>::align’ is too small to hold all values of ‘using align_t = enum fmt::v6::align::type {aka enum fmt::v6::align::type}’ [-Werror]

[peterbell10]

That's unfortunate; it looks like there isn't a nice way to satisfy both compilers. I can switch the struct data members to use uint8_t directly, but then we lose the type safety of enums.