refactor: plonk uses constraint/ and couple of fixes closes #467

[gbotrel]

This PR is based on top of @ivokub #481 and #483 .

frontend/cs/scs now uses constraint/ package (killed TermToRefactor and LinearExpressionToRefactor) .

builder.addPlonkConstraint(...) signature changed;

introduced

// qL⋅xa + qR⋅xb + qO⋅xc + qM⋅(xaxb) + qC == 0
type sparseR1C struct {
	xa, xb, xc         int              // wires
	qL, qR, qO, qM, qC constraint.Coeff // coefficients
}

and ensured all apis use this format clearly (doc + call).

Perf improvement:

• ~35% compile time for example for groth16 bls12377
• less constraints in some cases when calling api.Add(v1, v2, constant, v3, ...); the constant part is now "merged" into one of the sub-constraints (for example AssertIsLessOrEqual benefits from that).

Fixes

• some likely bugs in api.Xor api.Or and api.IsZero that would assume the coeff a variable that is boolean is always 1. (whereas we should check coeff*wire == 0|1).