Implement encodings

[claucece]

In accordance with Mike Hamburg's paper.

• [x] implement bigNumber arithmetic // merged
• [x] implement the 3 decaf new functions: equality, encoding and decoding // merged
• [ ] implement the new scalar operations
• [ ] implement the new point operations // check if all are done
• [x] change the point representation: projective coordinates
• [x] implement a generator for the base point according to the new coordinates // this should be on otr4
• [ ] implement elligator2
• [x] implement montgomery mul and use it on 'decode_long'

[yakiradixon]

Putting a note here to remind us that we discussed using an existing elligator implementation over implementing it ourselves

[claucece]

From scalar, missing:

• [x] Invert
• [ ] Set to unsigned 64
• [x] MontSquare
• [x] Destroy
• [x] CondSelect

From point, missing:

• [x] Point negate
• [x] Point dual scalar mul
• [x] Point torque
• [x] Point cond_sel

From pniels

• [ ] Check conversions
• [ ] prepare_fixed_window
• [ ] cond neg niels

Various

• [ ] Batch invert

Encodings:

• [x] x448
• [ ] eddsa: prehashed

Elligator:

• [ ] all