libstdc++: Implement _Escaping_sink to avoid construction of string

libstdc++: Implement _Escaping_sink to avoid construction of string

This patch implements _Escaping_sink that stores characters in a local (stack)
buffer. When the buffer is full, the range of characters is escaped and written
to the underlying sink.

To support above, the __write_escaped_unicode_part function are defined.
It takes __str and __prev_esc by reference. The __prev_esc value is updated
based on the last character written. If the buffer ends with an incomplete
code point sequence, __str is left non-empty and last code points are not
written. _Escaping_sink then copies these characters to the front of the
buffer to reconstruct the full code point.

__formatter__str::_M_format_range now uses _Escaping_sink to escape any
non-continuous character sequences.

libstdc++-v3/ChangeLog:

	* include/std/format (__format::__write_escape_seqs)
	(__format::_Escaping_sink): Define.
	(__format::__write_escaped_unicode_part): Extract from
	__format::__write_escaped_unicode.
	(__format::__write_escaped_unicode): Forward to
	__write_escaped_unicode_part.
	(__formatter_str::_M_format_range): Use _Escaping sink.
	* testsuite/std/format/ranges/string.cc: New tests for
	character which codepoints will be split in buffer and
	escaping. Invoked test_padding.

Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Signed-off-by: Tomasz Kamiński <tkaminsk@redhat.com>