Update demos

To build demos, run
`cmake .. -DENABLE_DEMOS=ON`

demos/src/sha256_digest_demo.c

@@ -0,0 +1,31 @@
+/*
+ *  Copyright 2014-2023 The GmSSL Project. All Rights Reserved.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the License); you may
+ *  not use this file except in compliance with the License.
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ */
+
+
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <gmssl/sha2.h>
+
+
+int main(int argc, char **argv)
+{
+	uint8_t dgst[SHA256_DIGEST_SIZE];
+	size_t i;
+
+	sha256_digest((uint8_t *)"abc", 3, dgst);
+
+	printf("sha256('abc') = ");
+	for (i = 0; i < sizeof(dgst); i++) {
+		printf("%02x", dgst[i]);
+	}
+	printf("\n");
+
+	return 0;
+}