Improve xchar support for std::bitset formatter and fix fill copying bug

[phprus]

Fixes #4285

Improve xchar support for std::bitset formatter and fix a bug when copying the fill from basic_specs.

[vitaut]

Merged, thanks!

[ja11sop]

It looks like this commit has broken other behaviour - the following code now segfaults:

Stream << fmt::format( "{}", fmt::styled( Line, Style ) ) << "\n";

will try and get more details and add an issue - I suggest reverting this for now as it breaks consistently across our entire codebase in code used in our test reporting.

[ja11sop]

to be more precise here:

#0  0x000055555568bbf3 in fmt::v11::detail::buffer<char>::append<char> (this=0x0, begin=begin@entry=0x7fffffffb400 "\033[34m", end=end@entry=0x7fffffffb405 "")
    at /home/user/_download/git_https_github.com__fmtlib_fmt.git/include/fmt/base.h:1797

[phprus]

@ja11sop
Thanks for the report!
I'll have a look tomorrow.

[phprus]

@ja11sop
Tests for fmt::styled pass without errors:

fmt/test/color-test.cc

Lines 53 to 58 in dad3237

	EXPECT_EQ(fmt::format("{}{}", fmt::styled("red", fg(fmt::color::red)),
	fmt::styled("bold", fmt::emphasis::bold)),
	"\x1b[38;2;255;000;000mred\x1b[0m\x1b[1mbold\x1b[0m");
	EXPECT_EQ(fmt::format("{}", fmt::styled("bar", fg(fmt::color::blue) |
	fmt::emphasis::underline)),
	"\x1b[4m\x1b[38;2;000;000;255mbar\x1b[0m");

Please provide the value ​​of the `Style`` variable that cause the crash.
Or provide minimum code sample, please.

[ja11sop]

Please see #4289 - yes I can see those tests must have passed since the PR was merged but I am not sure they quite match the simple case shown in the linked issue.

I admit that it seems odd this PR affected things but reviewing the stack trace and looking at the code tells me the backend for fmt is not trivial, plus without this PR we have no issue, with the PR we do.