lib secp256k1.cr v0.2.0

this release refactors the entire library and entangles some of the submodules exposed by the library.

this library now exposes the following modules (in logical order):

• Secp256k1: necessary constants and data structures
• Secp256k1::Core: the entire core mathematics behind the elliptic curve cryptography
• Secp256k1::Util: all tools for the handling of private-public key-pairs
• Secp256k1::Hash: implementation of various hashing algorithms for convenience
• Secp256k1::Signature: allows for signing messages and verifying signatures
• Secp256k1::Bitcoin: for the generation of bitcoin addresses
• Secp256k1::Ethereum: for the generation of ethereum addresses

docs are now published on github pages: q9f.github.io/secp256k1.cr

changes:

• add docs to readme (#26)
• add crystal docs (#25)
• rename utils to util (#24)
• refactor code with meaningful submodule namespaces (#23)