Handle the iterator protocol properly in `st.cache`

Summary

As part of our December plan to fix hashing, we are defining new hashing semantics. This issue covers step 3.

More background

This task is important becuase:

1. It means we no longer have to special case lists and tuples.
2. It is a prerequisite to enable hashing of other primitive types through default hash_funcs (INSERT LINK HERE).
3. It allows the user to recursively hash container types.

Details

Step 3 states that we handle iterators properly.

Roughly the semantics are:

try:
    for elem in iter(x):
        hash(elem)
except (this iter thing failed):
    go onto step 4

Important Note: When this is done, we no longer need to special case lists and tuples!

[tvst]

Ok, we’ve made the following decision:

• Let’s hash lists, tuples, and dicts
• Let’s not hash other kinds of iterables. These should show the unhashable warning.
• Let’s not hash generators. These should show the unhashable warning.

[treuille]

No. Not missing anything. Lemme think about this and get back to you!