crypto/secp256k1: fix undefined behavior in BitCurve.Add (#22621)

This commit changes the behavior of BitCurve.Add to be more inline
with btcd. It fixes two different bugs:

1) When adding a point at infinity to another point, the other point
   should be returned. While this is undefined behavior, it is better
   to be more inline with the go standard library.
   Thus (0,0) + (a, b) = (a,b)

2) Adding the same point to itself produced the point at infinity.
   This is incorrect, now doubleJacobian is used to correctly calculate it.
   Thus (a,b) + (a,b) == 2* (a,b) and not (0,0) anymore.

The change also adds a differential fuzzer for Add, testing it against btcd.

Co-authored-by: Felix Lange <fjl@twurst.com>

tests/fuzzers/secp256k1/secp_test.go

@@ -0,0 +1,8 @@
+package secp256k1
+
+import "testing"
+
+func TestFuzzer(t *testing.T) {
+	test := "00000000N0000000/R00000000000000000U0000S0000000mkhP000000000000000U"
+	Fuzz([]byte(test))
+}