scm_i_utf8_string_hash: compute u8 chars not bytes

Noticed while investigating a migration to utf-8 strings.  After making
changes that routed non-ascii symbol hashing through this function,
encoding-iso88597.test began intermittently failing because it would
traverse trailing garbage when u8_strnlen reported 8 chars instead of 4.

Change the scm_i_str2symbol and scm_i_str2uninterned_symbol internal
hash type to unsigned long to explicitly match the scm_i_string_hash
result type.

* libguile/hash.c (scm_i_utf8_string_hash): Call u8_mbsnlen not u8_strnlen.
* libguile/symbols.c (scm_i_str2symbol, scm_i_str2uninterned_symbol):
Use unsigned long for scm_i_string_hash result.
* test-suite/standalone/.gitignore: Add test-hashing.
* test-suite/standalone/Makefile.am: Add test-hashing.
* test-suite/standalone/test-hashing.c: Add.

test-suite/standalone/test-hashing.c

@@ -0,0 +1,63 @@
+/* Copyright 2022-2023
+     Free Software Foundation, Inc.
+
+   This file is part of Guile.
+
+   Guile is free software: you can redistribute it and/or modify it
+   under the terms of the GNU Lesser General Public License as published
+   by the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
+
+   Guile is distributed in the hope that it will be useful, but WITHOUT
+   ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+   FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
+   License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with Guile.  If not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#if HAVE_CONFIG_H
+# include <config.h>
+#endif
+
+#include <libguile.h>
+
+#include <stdio.h>
+
+static void
+test_hashing ()
+{
+  // Make sure a utf-8 symbol has the expected hash.  In addition to
+  // catching algorithmic regressions, this would have caught a
+  // long-standing buffer overflow.
+
+  // Περί
+  char about_u8[] = {0xce, 0xa0, 0xce, 0xb5, 0xcf, 0x81, 0xce, 0xaf, 0};
+  SCM sym = scm_from_utf8_symbol (about_u8);
+
+  // Value determined by calling wide_string_hash on {0x3A0, 0x3B5,
+  // 0x3C1, 0x3AF} via a temporary test program.
+  const unsigned long expect = 4029223418961680680;
+  const unsigned long actual = scm_to_ulong (scm_symbol_hash (sym));
+
+  if (actual != expect)
+    {
+      fprintf (stderr, "fail: unexpected utf-8 symbol hash (%lu != %lu)\n",
+               actual, expect);
+      exit (EXIT_FAILURE);
+    }
+}
+
+static void
+tests (void *data, int argc, char **argv)
+{
+  test_hashing ();
+}
+
+int
+main (int argc, char *argv[])
+{
+  scm_boot_guile (argc, argv, tests, NULL);
+  return 0;
+}