btcec: Optimize field inversion.

[bmperrea]

This uses the addition chain from Brian Smith's website to speed up the inversion algorithm for btcec/field.go.

[bmperrea]

It gets back the sig-verify speed lost in #1084 😄 .

benchmark                          old ns/op     new ns/op     delta
----------------------------------------------------------------------
BenchmarkAddJacobian-8             550           552           +0.36%
BenchmarkAddJacobianNotZOne-8      1124          1128          +0.36%
BenchmarkScalarBaseMult-8          49320         48393         -1.88%
BenchmarkScalarBaseMultLarge-8     50016         48749         -2.53%
BenchmarkScalarMult-8              160763        158805        -1.22%
BenchmarkNAF-8                     867           870           +0.35%
BenchmarkSigVerify-8               256511        251033        -2.14%
BenchmarkFieldNormalize-8          13.7          13.7           0.00%

[bmperrea]

@davecgh Any chance we can get this into 0.13.0?

[jakesylvestre]

@jcvernaleo (as per #1530)

• Low priority
• Enhancement