Wrap BN_bn2bin_padded() which comes useful for exporting fixed-length
BIGNUMs, more efficient than padding result of to_vec() afterwards.
Note that in OpenSSL the function is called BN_bn2binpad() and has
a different order of arguments. BoringSSL's BN_bn2bin_padded() also
takes the desired length as "size_t".
In particular, this updates `foreign-types`, which had a lot of breaking changes.
- `ForeignType` is now an unsafe trait
- `*Ref` types no longer need a separate macro call, they're generated automatically
- Generated types now store `NonNull<T>` instead of `*mut T`
Kornel