Fix terminology in code

[zcash]

Fix terminology in code

[therealyingtong]

best_multiexp -> best_msm

[therealyingtong]

additionally:

• LagrangeCoeff -> LagrangeBasis,
• ExtendedLagrangeCoeff -> ExtendedLagrangeBasis,
• Coeff -> MonomialBasis

[daira]

• aux -> public or external instance (see #181)

(because "auxiliary input" is one way of saying a private input, i.e. an advice column).

[daira]

Rename the challenge x_2 to x_7 values. Make sure we use the same names in the Halo 2 write-up/paper as in the code.

[daira]

• gate -> rule
• index -> neighbour, for a relative cell position (e.g. in query_fixed_index)

Both of these match the cellular automaton analogy. (Note that in that terminology, neighbour doesn't necessarily mean an immediately adjacent cell, just a cell that participates in a rule.) I'm also fine with neighbor; I don't know whether we use British or American spelling for other things?