mobile: Add Sign to mobile BigInt

Currently, the only way of getting the sign of a BigInt from mobile is
coverting to string. This adds a Sign function to make conversion
faster.

mobile/big.go

@@ -62,6 +62,16 @@ func (bi *BigInt) SetInt64(x int64) {
 	bi.bigint.SetInt64(x)
 }
 
+// Sign returns:
+//
+//	-1 if x <  0
+//	 0 if x == 0
+//	+1 if x >  0
+//
+func (bi *BigInt) Sign() int {
+	return bi.bigint.Sign()
+}
+
 // SetString sets the big int to x.
 //
 // The string prefix determines the actual conversion base. A prefix of "0x" or