LoopyKeyBuilder: hash for BasicSet differs for objects that compare equal

```python
import islpy as isl

a=isl.BasicSet("[im_w, im_h, nimgs, nfeats] -> { : im_w >= 7 and im_h >= 7 and nimgs >= 0 and nfeats > 0 }")

b=isl.BasicSet("[nfeats, nimgs, im_h, im_w] -> { : nfeats > 0 and nimgs >= 0 and im_h >= 7 and im_w >= 7 }")

from loopy.tools import LoopyKeyBuilder

assert a == b
assert LoopyKeyBuilder()(a) == LoopyKeyBuilder()(b)
```

```console
$ python set.py
Traceback (most recent call last):
File "/Users/mdiener/Work/e12test/loopy/set.py", line 12, in
assert LoopyKeyBuilder()(a) == LoopyKeyBuilder()(b)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError
```

The same issue exists with `hash()`, as well as `Set`(including `Set.get_hash`).

cc: #576, #828, https://github.com/inducer/islpy/issues/89

Start with loopy.tools.LoopyKeyBuilder and inspect how it hashes isl.BasicSet and Set objects, including Set.get_hash; the supplied reproducer shows the failing case. Compare the hash paths for equal objects and use that reproducer to verify that equal BasicSet and Set values produce equal hashes.

Written by the indexing model from the issue text.