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.