lib secp256k1.cr v0.2.1

this release significantly improves the documentation at https://q9f.github.io/secp256k1.cr/ and applies some minor enhancements to the exposed apis of the library, namely:

• mini-private keys now use BASE-56 instead of BASE-57. that does not affect existing mini-private keys.
• added Secp256k1::Bitcoin.wif_from_private_uncompressed wrapper for uncompressed wif
• renamed Secp256k1::Bitcoin.wif_compressed_from_private to .wif_from_private_compressed
• private keys are handled as BigInt instead of a String literals wherever possible, e.g., Secp256k1::Bitcoin.address_from_private or Secp256k1::Ethereum.address_from_private
• constants: renamed EC_PARAM_PRIME to EC_PRIME_P
• replaced BASE_57 in favor of BASE_56 by removing the o from the alphabet as recommended by the Bitcoin Wiki.
• binary hashing operations now directly operate on Bytes instead of String literals, e.g., Secp256k1::Hash.sha3, .sha256, .keccak256, .ripemd160
• added Secp256k1::Hash.ripemd160_string for RIPEMD-160 hashes on String literals to complement .sha3_string, .sha256_string, and .keccak256_string
• added Secp256k1::Hash.base58_char to complement .base56_char
• pass the prime through Secp256k1::Util.restore_public_key

changes:

• improve the documentation #27