Remove iostream dependency

[daniel-starke]

The current implementation uses std::ostream in detail::print_parser() and std::stringstream in parser.call() for error reporing. For static linked executables this brings in the whole locale mashinary of STL. Something not always desirable. I suggest replacing it or at least make it customizable whether to use iostreams or not.

I have made two examples with such a parser using printf() for error reporting:
lit('{') > int_ > lit(',') > int_ > lit('}')

Boost Spirit X3 results in a 154 KiB executable and Boost Parser in 218 KiB with error reporting removed in parser.hpp and 963 KiB with error reporting included.
Both compiled with GCC 15.1 for MinGW-W64 using -O2 -ffunction-sections -fdata-sections -s -static -Wl,--gc-sections. LTO makes hardly any difference here.

[tzlaine]

I intend to do this once std::format is more widely available. At the time I started this project, it wasn't implemented fully on any of the major implementations. I won't be doing it soon, probably, but I do intend to address this in future. Now that I know someone might want to use a non-iostream, non-format approach, I'll consider making that one of the customization options.