[iter] Use static_assert with hb_is_random_access_iterator()

[iter] Use static_assert with hb_is_random_access_iterator()

Both, checks constexpr'ness, and fixes build with cra**y implementations
of assert() macro:

test-iter.cc:108:11: error: too many arguments provided to function-like macro invocation
  assert (hb_is_random_access_iterator (array_iter_t<int>));
          ^
./hb-iter.hh:186:42: note: expanded from macro 'hb_is_random_access_iterator'
  hb_is_random_access_iterator_of (Iter, typename Iter::item_t)
                                         ^
/usr/include/x86_64-linux-gnu/sys/cdefs.h:89:9: note: macro '__STRING' defined here
\#define __STRING(x)     #x
        ^
test-iter.cc:108:3: error: use of undeclared identifier '__STRING'
  assert (hb_is_random_access_iterator (array_iter_t<int>));
  ^
/usr/include/assert.h:91:21: note: expanded from macro 'assert'
   : __assert_fail (__STRING(expr), __FILE__, __LINE__, __ASSERT_FUNCTION))
                    ^