crypto/secp256k1: update to github.com/bitcoin-core/secp256k1 @ 9d560f9 (#3544)

- Use defined constants instead of hard-coding their integer value.
- Allocate secp256k1 structs on the C stack instead of converting []byte
- Remove dead code

crypto/secp256k1/libsecp256k1/src/field_5x52_impl.h

@@ -11,7 +11,6 @@
 #include "libsecp256k1-config.h"
 #endif
 
-#include <string.h>
 #include "util.h"
 #include "num.h"
 #include "field.h"
@@ -50,10 +49,6 @@ static void secp256k1_fe_verify(const secp256k1_fe *a) {
     }
     VERIFY_CHECK(r == 1);
 }
-#else
-static void secp256k1_fe_verify(const secp256k1_fe *a) {
-    (void)a;
-}
 #endif
 
 static void secp256k1_fe_normalize(secp256k1_fe *r) {