Move the hasher to the inner HashMap

[cuviper]

This is an alternate to #21 that gives HashMap control of most hashing, by introducing a private NodeKey<K, V> type around the node pointer that only looks at K for equality and hashing. A private NodeQ wrapper also works to enable the generic K: Borrow<Q> relationship while having NodeKey in the middle.

As with #21, LinkedHashMap::reserve and try_reserve are now constrained such that K can be (re)hashed with S, which is needed when hashbrown tries to move them to a new allocation. The new test_reserve demonstrates a case that would have previously failed.

In addition, OccupiedEntry and RawOccupiedEntryMut now carry the S parameter as well, since hashbrown's entry type needs it now that it's not just NullHasher (which has been removed).

[kyren]

Closed in favor of #21