Rename tor_mem_is_zero to fast_mem_is_zero()

For memeq and friends, "tor_" indicates constant-time and "fast_"
indicates optimized.  I'm fine with leaving the constant-time
"safe_mem_is_zero" with its current name, but the "tor_" prefix on
the current optimized version is misleading.

Also, make the tor_digest*_is_zero() uniformly constant-time, and
add a fast_digest*_is_zero() version to use as needed.

A later commit in this branch will fix all the users of
tor_mem_is_zero().

Closes ticket 30309.

src/lib/string/util_string.h

@@ -20,7 +20,13 @@ const void *tor_memmem(const void *haystack, size_t hlen, const void *needle,
                        size_t nlen);
 const void *tor_memstr(const void *haystack, size_t hlen,
                        const char *needle);
-int tor_mem_is_zero(const char *mem, size_t len);
+int fast_mem_is_zero(const char *mem, size_t len);
+#define fast_digest_is_zero(d) fast_mem_is_zero((d), DIGEST_LEN)
+#define fast_digetst256_is_zero(d) fast_mem_is_zero((d), DIGEST256_LEN)
+
+// XXXX remove this after we replace all users.
+#define tor_mem_is_zero fast_mem_is_zero
+
 int tor_digest_is_zero(const char *digest);
 int tor_digest256_is_zero(const char *digest);