3-coin crypto pool

[chanhosuh]

Closes #91

The goal is to do only what is required to get reliable crypto sims going, as we expect there will be a lot of work to tune the sims to reflect accurate repegging and other timing issues.

This means that cryptopools only need:

• exchange
• tweak_price
• dydx

The SimPool wrapper will only need these to do crypto sims.

Some further work remains:

• cleanup code, including branching on n_coins (if possible)
• fully testing exchange under more conditions, especially swap sequences, and checking repegs
• Speedup calcs using gmpy2
• increase coverage
• changelog entry

Bonus!

• Parallelized pytest (shaving 5 minutes off(!) to go from around 16 minutes total CI time to around 11)
  (Cherry-picked into main)

[github-actions]

Coverage report

The coverage rate went from 74.67% to 75.29% ⬆️
The branch rate is 63%.

84.57% of new lines are covered.

Diff Coverage details (click to unfold)

curvesim/pool/cryptoswap/pool.py

85.55% of new lines are covered (83.93% of the complete file).
Missing lines: 157, 178, 189, 201, 202, 203, 205, 322, 449, 589, 695, 715, 716, 717, 719, 721, 722, 723, 725, 726, 728, 928, 1009, 1026, 1038, 1060

curvesim/pool/cryptoswap/calcs/factory_2_coin.py

76.05% of new lines are covered (70.2% of the complete file).
Missing lines: 40, 47, 48, 50, 51, 52, 54, 55, 56, 57, 58, 60, 67, 68, 88, 90, 111, 112, 113, 114, 115, 116, 138, 139, 152, 162, 181, 183, 203, 204, 205, 230, 239, 242

curvesim/pool/cryptoswap/calcs/tricrypto_ng.py

87.94% of new lines are covered (84% of the complete file).
Missing lines: 107, 109, 111, 113, 115, 127, 129, 130, 131, 132, 133, 141, 146, 152, 158, 166, 172, 178, 182, 224, 226, 243, 244, 245, 274, 275, 288, 298, 310, 324, 417, 421, 501, 512, 519, 556, 608

[chanhosuh]

Looks like the _newton_D will need to be updated with an optional starting guess argument. This will make the price tweak match the Tricrypto-NG more precisely. Otherwise there starts to be a drift in the price scale which shows up in slight changes in tolerance required for swap comparisons.