When printing into a buffer, generate all the text

When printing into a buffer, generate all the text
first, then insert it all at once.
(print_buffer): New variable.
(print_buffer_size, print_buffer_pos): New variables.
(PRINTPREPARE): Allocate print_buffer.
(PRINTFINISH): Free print_buffer after inserting its contents.
(printchar, strout): Output into print_buffer.
(print_string): If printcharfun is nil, use strout.