Make format_pieces recognize the \e escape sequence

I noticed that the printf command did not recognize the \e escape
sequence, used amongst other things to use colors:

  (gdb) printf "This is \e[32mgreen\e[m!\n"
  Unrecognized escape character \e in format string.

This patch makes format_pieces recognize it, which makes that command
print the expected result in glorious color.

I wrote a really simple unit test for format_pieces.
format_pieces::operator[] is unused so I removed it.  I added
format_piece::operator==, which is needed to compare vectors of
format_piece.

gdb/ChangeLog:

	PR cli/14975
	* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
	unittests/format_pieces-selftests.c.
	* common/format.h (format_piece) <operator==>: New.
	(format_pieces) <operator[]>: Remove.
	* common/format.c (format_pieces::format_pieces): Handle \e.
	* unittests/format_pieces-selftests.c: New.