Implementation of the Jubjub elliptic curve group and its associated fields.

This is a fork of the original jubjub crate from Zcash. The fork was created by the Iron Fish project to add performance improvements.

Delta from upstream

These are the differences between this crate and the upstream jubjub crate:

• Changed the elliptic curve backend from bls12_381 to blstrs
• Added optional statistics counters for performance monitoring. This can be enabled through the stats feature.
• Added a ExtendedPoint::multiply_many method to efficiently perform multiple elliptic curve multiplications in one call.
• Added a SubgroupPoint::as_extended method to convert a SubgroupPoint to an ExtendedPoint.