Make input types convertible to data_slice

This reverts commit 8837fcd10cec8861de2a3e97f4c5653b7fb97e29.
"Quick fix to accomodate libbitcoin array_slice signature changes."
Use btc256 instead of base16 for hashes
Adapt to hash changes
Merge pull request #171 from libbitcoin/reverse

Base16
Build secpk256k1 from libbitcoin fork.
Add sodium dynamic option for with-pkgconfigdir.
Fix typo.