Efficient parsing

Added

• Parsing of data structures is now way more efficient
• Opcodes have been isolated in a separate file and helper methods have been created to deal with them

Fixed

• Parsing of peculiar scripts no longer crashes (e.g. empy push operations, multisig scripts without public keys)

Backward compatibility

Breaks the following functionality:

• Any code that used the following static fields of the BaseScript class now needs to be refactored to use the opcodes module:
  • opcode_to_int
  • int_to_opcode
  • opcode_to_hex
  • hex_to_opcode