just like i2osp, but take an extra parameter for size.
if the number is too big to fit in
len bytes, nothing is returned
otherwise the number is padded with 0 to fit the len required.
FIXME: use unsafeCreate to fill the bytestring
just like i2ospOf except that it doesn't expect a failure. for example if you just took a modulo of the number that represent the size (example the RSA modulo n).