Introduce hash context to support pluggable SHA256 compression

This is purely a mechanical change with no behavior change.

It introduces a secp256k1_hash_ctx struct inside secp256k1_context
and propagates it to all SHA256-related operations.

This sets up the ability to provide a hardware-optimized SHA256
compression function at runtime in a follow-up commit.

src/modules/ecdh/tests_impl.h

@@ -83,8 +83,8 @@ static void test_ecdh_generator_basepoint(void) {
         /* compute "explicitly" */
         CHECK(secp256k1_ec_pubkey_serialize(CTX, point_ser, &point_ser_len, &point[1], SECP256K1_EC_COMPRESSED) == 1);
         secp256k1_sha256_initialize(&sha);
-        secp256k1_sha256_write(&sha, point_ser, point_ser_len);
-        secp256k1_sha256_finalize(&sha, output_ser);
+        secp256k1_sha256_write(secp256k1_get_hash_context(CTX), &sha, point_ser, point_ser_len);
+        secp256k1_sha256_finalize(secp256k1_get_hash_context(CTX), &sha, output_ser);
         /* compare */
         CHECK(secp256k1_memcmp_var(output_ecdh, output_ser, 32) == 0);
     }