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/libsecp256k1.pc.in

@@ -5,7 +5,7 @@ includedir=@includedir@
 
 Name: libsecp256k1
 Description: Optimized C library for EC operations on curve secp256k1
-URL: https://github.com/bitcoin/secp256k1
+URL: https://github.com/bitcoin-core/secp256k1
 Version: @PACKAGE_VERSION@
 Cflags: -I${includedir}
 Libs.private: @SECP_LIBS@