Replace spl_object_hash() with spl_object_id()

[thekid]

Quoting https://externals.io/message/131452#131487:

Is spl_object_hash() still useful? The string it creates used to be
quite a bit more complex, but eventually it became equivalent to

str_pad(dechex(spl_object_id($obj)), 16, '0', STR_PAD_LEFT).'0000000000000000'

I.e., it provides exactly the same content as spl_object_id, but in a
much more obtuse way. For anyone needing that format, the polyfill is
just above.

Forward compatibility, see #368