Add `BigNum::copy_from_slice`

Unfortunately `BigNum::from_slice` always creates a new BigNum and
cannot be used to initialize an already existing BigNum thus it is not
possible to have a secure BigNum initialized from a slice.

This patch adds the function for overwriting existing BigNum with given
slice bytes. Thus the BigNum can be created using `BigNum::new_secure`
and then overwritten using `BigNum::copy_from_slice`.