Add fmt_print and fmt_println to C API

[Ferdi265]

This PR introduces fmt_print() and fmt_println() macros to the C API, which print to a FILE* stream using fmt::vprint(stream, fmt, format_args).

Additionally, this PR introduces the following helpers in fmt-c.h:

• a helper macro FMT_FORMAT_ARGS(fmt, ...) that abstracts away the macro magic necessary to produce the fmt_arg array, useful for avoiding duplicated logic when declaring additional wrapper macros that expand to calls to fmt_vformat or similar
• a fmt_vprint() function used to implement the fmt_print() macro
• a fmt_vprintln() function used to implement the fmt_println() macro

and the following helpers in fmt-c.cc:

• a helper template fmt_c_wrapper() that performs the conversion from fmt_arg to fmt::basic_format_arg, calls the passed lambda, and handles exceptions, to reduce boilerplate and duplication between fmt_vformat(), fmt_vprint(), and fmt_vprintln()