Modify pairings API to support various serialization formats

[lpetrovic05]

Pairings elements will need to be serialized to various formats. The API should allow for different serialization options. The following data should be exposed:

• Field elements:
  • Just a number, as a byte array (big endian)
• Group elements:
  • Zero element boolean
  • X coordinate (big endian)
  • Y coordinate (big endian)
  • Is Y negative boolean

Also deprecate the toBytes and fromBytes methods.