lib/string: add `chomp`

Signed-off-by: Jean Privat <jean@pryen.org>
lib/stream: add IStream::read_lines

Signed-off-by: Jean Privat <jean@pryen.org>

read_lines
lib/stream: document `read_line` and `append_line_to`

Signed-off-by: Jean Privat <jean@pryen.org>