Hey,

I've finished migrating the script class away from OpenSSL.

https://github.com/libbitcoin/libbitcoin/blob/develop/include/bitcoin/utility/script_number.hpp

Unit test:

https://github.com/libbitcoin/libbitcoin/blob/develop/test/scriptnum.cpp

Remaining usage of bignum is simply for proof of work calculations:

* Adding hashes together to calculate total work.
* Performing comparisons with targets.
* Difficulty readjustment (see validate.cpp).

These shouldn't be too difficult but now the major part is done, I'm
going to focus a bit on the blockchain.