io: added stream status and seeking.  file_io: corrected log messages.

README: clarified POSIX/C stdlib usage (via the C++ stdlib).

io:
	added IOStatus for stream operations;
	added status(), position(), seek_to(), and seek_relative();
	removed by-reference read_value();
	added IOStatus return values for IO operations.

file_io: added newline to log messages (oops).

test/io/file_io: added status, position, and seek assertions.
impl/file_io/posix: added binary mode to fopen() calls.¹

1: This doesn't even matter on Linux, but I feel it should be
explicit. Also, I suspect this impl will merge/rename to a
general C impl as it should be supported on other platforms.
Won't know until I explore those platforms.