Merge bitcoin-core/secp256k1#1824: util: introduce and use ARRAY_SIZE macro

921b9711ea util: introduce and use `ARRAY_SIZE` macro (Sebastian Falbesoner)

Pull request description:

  This PR is another tiny improvement found while working on #1765, with the goal to avoid code repetition.

  The `ARRAY_SIZE` macro definition is pretty wide-spread in C projects and e.g. matches the one [used in the Linux Kernel](9702969978/include/linux/array_size.h (L11))  (without the additional check to reject pointers, as we would need GNU C for that, see e.g. https://stackoverflow.com/a/19455169; not sure if a useful counterpart exists that only relies on C89). Replacement instances were identified via `$ git grep sizeof.*/.*sizeof`.

ACKs for top commit:
  w0xlt:
    ACK 921b9711ea
  real-or-random:
    utACK 921b9711ea

Tree-SHA512: 44b6bf0132cf00fade526a3fc04e03dc896d04874123614c032206b61f97c81f94d139b6cc0c108eceaa699251580c19420d230b3150607303ca2cb7ab9a0bcb

src/bench.c

@@ -183,8 +183,7 @@ int main(int argc, char** argv) {
                          "ecdsa_recover", "schnorrsig", "schnorrsig_verify", "schnorrsig_sign", "ec",
                          "keygen", "ec_keygen", "ellswift", "encode", "ellswift_encode", "decode",
                          "ellswift_decode", "ellswift_keygen", "ellswift_ecdh"};
-    size_t valid_args_size = sizeof(valid_args)/sizeof(valid_args[0]);
-    int invalid_args = have_invalid_args(argc, argv, valid_args, valid_args_size);
+    int invalid_args = have_invalid_args(argc, argv, valid_args, ARRAY_SIZE(valid_args));
 
     int default_iters = 20000;
     int iters = get_iters(default_iters);