lib secp256k1.cr v0.3.0

this minor release improves stability. the following api renames shall be noted:

• EC_Point is now ECPoint
• ECDSA_Signature is now ECDSASignature
• all hashing functions ending with _string are renamed now overloadable with String

changes:

• changes to code style (#34)
• add stats to nightly workflows (#36)
• overload hashing functions with strings (#37)
• update contributors (#38)