Add entry into_map methods

[cuviper]

This adds methods to get the original &'a mut HashMap reference from
hash_map::{Entry, EntryRef, OccupiedEntry, VacantEntry, VacantEntryRef}
to help with current borrow-checker limitations in returning lifetimes
from different branches.

    /// Converts the `Entry` into a mutable reference to the underlying map.
    pub fn into_map(self) -> &'a mut HashMap<K, V, S, A>;

There are already similar into_table methods on hash_table occupied
and vacant entries, but this also adds that to hash_table::Entry for
consistency.

This idea was raised in an internals thread for the standard
library, but it's not possible there since those entries don't have any
reference to return, especially since they aren't even parameterized by
the S hasher at all -- they are wrapping RustcEntry here.

[cuviper]

I also considered this for hash_set entries, i.e. fn into_set(self) -> &'a mut HashSet, but with the way they're wrapping the map entries, we would need to "upcast" the into_map reference to a set reference. That's possible, but I think we'd need to add #[repr(transparent)] to HashSet.

[clarfonthey]

So, main concern here is the hasher, and the fact that the table doesn't have one. So, if we were to store a pointer to the table in its entries and want to make the map implemented based upon those, we'd have to probably either do repr(C) or do some nonsense with offset_of to avoid storing two pointers.

But like, it's not a dealbreaker, just something to consider.

[cuviper]

Even if HashMap is built on HashTable (#666), these entries could keep &mut HashMap and a bucket index here, rather than wrapping the table entry types.

[clarfonthey]

Fair! I guess that bucket indices are part of that API, now, so, it would work pretty well.