secp256k1: No allocs in slow scalar base mult path.

This moves the conversion of the base point represented in Jacobian
coordinates outside of the slow scalar base multiplication path used in
resource constrained environments to avoid unnecessary allocations.

name                         old time/op    new time/op    delta
--------------------------------------------------------------------------------
ScalarBaseMultNonConstSlow   91.9µs ± 1%    91.5µs ± 1%    ~     (p=0.280 n=10+10)

name                         old alloc/op   new alloc/op   delta
--------------------------------------------------------------------------------
ScalarBaseMultNonConstSlow   64.0B ± 0%     0.0B           -100.00%  (p=0.000 n=10+10)

name                         old allocs/op  new allocs/op  delta
--------------------------------------------------------------------------------
ScalarBaseMultNonConstSlow   2.00 ± 0%      0.00           -100.00%  (p=0.000 n=10+10)

dcrec/secp256k1/curve.go

@@ -1224,11 +1224,17 @@ func ScalarBaseMultNonConst(k *ModNScalar, result *JacobianPoint) {
 	scalarBaseMultNonConst(k, result)
 }
 
-// scalarBaseMultNonConstSlow computes k*G through ScalarMultNonConst.
-func scalarBaseMultNonConstSlow(k *ModNScalar, result *JacobianPoint) {
+// jacobianG is the secp256k1 base point converted to Jacobian coordinates and
+// is defined here to avoid repeatedly converting it.
+var jacobianG = func() JacobianPoint {
 	var G JacobianPoint
 	bigAffineToJacobian(curveParams.Gx, curveParams.Gy, &G)
-	ScalarMultNonConst(k, &G, result)
+	return G
+}()
+
+// scalarBaseMultNonConstSlow computes k*G through ScalarMultNonConst.
+func scalarBaseMultNonConstSlow(k *ModNScalar, result *JacobianPoint) {
+	ScalarMultNonConst(k, &jacobianG, result)
 }
 
 // scalarBaseMultNonConstFast computes k*G through the precomputed lookup