The std::to_chars functions from C++17 can be used to implement
std::to_string with much better performance than calling snprintf. Only
the __detail::__to_chars_len and __detail::__to_chars_10 functions are
needed for to_string, because it always outputs base 10 representations.

The return type of __detail::__to_chars_10 should not be declared before
C++17, so the function body is extracted into a new function that can be
reused by to_string and __detail::__to_chars_10.

The existing tests for to_chars rely on to_string to check for correct
answers. Now that they use the same code that doesn't actually ensure
correctness, so add new tests for std::to_string that compare against
printf output.

	* include/Makefile.am: Add new <bits/charconv.h> header.
	* include/Makefile.in: Regenerate.
	* include/bits/basic_string.h (to_string(int), to_string(unsigned))
	(to_string(long), to_string(unsigned long), to_string(long long))
	(to_string(unsigned long long)): Rewrite to use __to_chars_10_impl.
	* include/bits/charconv.h: New header.
	(__detail::__to_chars_len): Move here from <charconv>.
	(__detail::__to_chars_10_impl): New function extracted from
	__detail::__to_chars_10.
	* include/std/charconv (__cpp_lib_to_chars): Add, but comment out.
	(__to_chars_unsigned_type): New class template that reuses
	__make_unsigned_selector_base::__select to pick a type.
	(__unsigned_least_t): Redefine as __to_chars_unsigned_type<T>::type.
	(__detail::__to_chars_len): Move to new header.
	(__detail::__to_chars_10): Add inline specifier. Move code doing the
	output to __detail::__to_chars_10_impl and call that.
	* include/std/version (__cpp_lib_to_chars): Add, but comment out.
	* testsuite/21_strings/basic_string/numeric_conversions/char/
	to_string.cc: Fix reference in comment. Remove unused variable.
	* testsuite/21_strings/basic_string/numeric_conversions/char/
	to_string_int.cc: New test.

From-SVN: r272186