Merge bitcoin-core/secp256k1#1748: bench: improve context creation in ECDH benchmark

dfe284ed2d bench: improve context creation in ECDH benchmark (Sebastian Falbesoner)

Pull request description:

  Calling `secp256k1_context_create` with `SECP256K1_FLAGS_TYPE_CONTEXT` seems to be confusing and not strictly API-compliant, as the only allowed (non-deprecated) value is `SECP256K1_CONTEXT_NONE`, even if the former happens to map to the latter currently.

  Fix this by not dynamically creating a context in the first place and switch to using the static context, as it is sufficient for this benchmark and presumably matches what the "no capabilities" comment intended back then.

  Alternatives are:
  * keep the signing context and only fix the name, i.e. s/_FLAGS_TYPE_CONTEXT/_CONTEXT_NONE/
  * use `secp256k1_context_static` everywhere directly and get rid of the `ctx` field in the `bench_ecdh_data` struct (less flexible for future changes, deviates from other bench structures)

  Found while reviewing #1698, see https://github.com/bitcoin-core/secp256k1/pull/1698#discussion_r2350395913.

ACKs for top commit:
  sipa:
    utACK dfe284ed2d
  stratospher:
    ACK dfe284e. not sure whether the alternative is preferred though.
  real-or-random:
    utACK dfe284ed2d
  furszy:
    ACK dfe284ed2d

Tree-SHA512: 106d115ec11577fcd66e93293289545aa0fa78480f4bdc8c440963e4e6b050c81d4775268cfa0e1ab44db4c08a5768a0ff80f74b866e550ddd22a4e17ccb9014

src/modules/ecdh/bench_impl.h

@@ -10,7 +10,7 @@
 #include "../../../include/secp256k1_ecdh.h"
 
 typedef struct {
-    secp256k1_context *ctx;
+    const secp256k1_context *ctx;
     secp256k1_pubkey point;
     unsigned char scalar[32];
 } bench_ecdh_data;
@@ -46,12 +46,9 @@ static void run_ecdh_bench(int iters, int argc, char** argv) {
     bench_ecdh_data data;
     int d = argc == 1;
 
-    /* create a context with no capabilities */
-    data.ctx = secp256k1_context_create(SECP256K1_FLAGS_TYPE_CONTEXT);
+    data.ctx = secp256k1_context_static;
 
     if (d || have_flag(argc, argv, "ecdh")) run_benchmark("ecdh", bench_ecdh, bench_ecdh_setup, NULL, &data, 10, iters);
-
-    secp256k1_context_destroy(data.ctx);
 }
 
 #endif /* SECP256K1_MODULE_ECDH_BENCH_H */