add NEON support to blake3_dispatch.c

Currently this requires setting the BLAKE3_USE_NEON preprocessor flag.
In the future we may enable this automatically on AArch32/64 or include
some kind of dynamic feature detection. (Though ARM makes this harder
than x86.)

Closes #30.

c/blake3_dispatch.c

@@ -275,6 +275,13 @@ void blake3_hash_many(const uint8_t *const *inputs, size_t num_inputs,
   }
 #endif
 #endif
+
+#if defined(IS_ARM) && defined(BLAKE3_USE_NEON)
+  blake3_hash_many_neon(inputs, num_inputs, blocks, key, counter,
+                        increment_counter, flags, flags_start, flags_end, out);
+  return;
+#endif
+
   blake3_hash_many_portable(inputs, num_inputs, blocks, key, counter,
                             increment_counter, flags, flags_start, flags_end,
                             out);