sha256: add sha256_direct_hex() helper

A various places we need a SHA256SUM of something as a string. Let's add
a simple helper for this that does this generically.

src/shared/pcrextend-util.c

@@ -358,7 +358,7 @@ int pcrextend_imds_userdata_word(const struct iovec *data, char **ret) {
         /* We include both a hash of the complete user data, and a truncated version of the data in the word
          * we measure. The former protects the actual data, the latter is useful for debugging. */
 
-        _cleanup_free_ char *hash = hexmem(SHA256_DIRECT(data->iov_base, data->iov_len), SHA256_DIGEST_SIZE);
+        _cleanup_free_ char *hash = sha256_direct_hex(data->iov_base, data->iov_len);
         if (!hash)
                 return log_oom();
 

src/shared/tar-util.c

@@ -14,7 +14,6 @@
 #include "chattr-util.h"
 #include "fd-util.h"
 #include "fs-util.h"
-#include "hexdecoct.h"
 #include "iovec-util.h"
 #include "libarchive-util.h"
 #include "mountpoint-util.h"
@@ -1184,7 +1183,7 @@ static int hardlink_lookup(
         else
                 assert(d->have_unique_mount_id == (r > 0));
 
-        m = hexmem(SHA256_DIRECT(handle->f_handle, handle->handle_bytes), SHA256_DIGEST_SIZE);
+        m = sha256_direct_hex(handle->f_handle, handle->handle_bytes);
         if (!m)
                 return log_oom();