Implement encodings

[claucece]

In accordance with Mike Hamburg's paper.

• implement bigNumber arithmetic // merged
• 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
• change the point representation: projective coordinates
• implement a generator for the base point according to the new coordinates // this should be on otr4
• implement elligator2
• 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:

• Invert
• Set to unsigned 64
• MontSquare
• Destroy
• CondSelect

From point, missing:

• Point negate
• Point dual scalar mul
• Point torque
• Point cond_sel

From pniels

• Check conversions
• prepare_fixed_window
• cond neg niels

Various

• Batch invert

Encodings:

• x448
• eddsa: prehashed

Elligator:

• all