std: count hash_map tombstones as available

When entries are inserted and removed into a hash map at an equivalent rate (maintaining a mostly-consistent total count of entries), it should never need to be resized. But `HashMapUnmanaged.available` does not presently count tombstoned slots as "available", so this put/remove pattern eventually panics (assertion failure) when `available` reaches `0`.

The solution implemented here is to count tombstoned slots as "available". Another approach (which hashbrown (https://github.com/rust-lang/hashbrown/blob/b3eaf32e608d1ec4c10963a4f495503d7f8a7ef5/src/raw/mod.rs#L1455-L1542) takes) would be to rehash all entries in place when there are too many tombstones. This is more complex but avoids an `O(n)` bad case when the hash map is full of many tombstones.

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