py, unix: Allow to compile with -Wunused-parameter.

See issue #699.
diff --git a/py/objstrunicode.c b/py/objstrunicode.c
index fd36787..49eddcc 100644
--- a/py/objstrunicode.c
+++ b/py/objstrunicode.c
@@ -114,6 +114,8 @@
 }
 
 STATIC mp_obj_t str_make_new(mp_obj_t type_in, mp_uint_t n_args, mp_uint_t n_kw, const mp_obj_t *args) {
+    (void)type_in;
+
 #if MICROPY_CPYTHON_COMPAT
     if (n_kw != 0) {
         mp_arg_error_unimpl_kw();
@@ -160,6 +162,7 @@
 // be capped to the first/last character of the string, depending on is_slice.
 const byte *str_index_to_ptr(const mp_obj_type_t *type, const byte *self_data, mp_uint_t self_len,
                              mp_obj_t index, bool is_slice) {
+    (void)type;
     mp_int_t i;
     // Copied from mp_get_index; I don't want bounds checking, just give me
     // the integer as-is. (I can't bounds-check without scanning the whole