Remove large amounts of unsafe from Refs via 'detached' lock guards

[conradludgate]

(stacked on top of #322)

I came up with this novel abstraction for reducing the dependencies on pointers within dashmap. We need to hold the lock guard for the shard.

Since Ref/Mut wants to get proper reference values eventually, it doesn't matter whether you convert pointers early or later. It seems to work all the same.

Thus, we can convert

pub struct Ref<'a, K, V> {
    _guard: RwLockReadGuard<'a, HashMap<K, V>>,
    k: *const K,
    v: *const V,
}

into

pub struct Ref<'a, K, V> {
    _guard: RwLockReadGuardDetached<'a>,
    k: &'a K,
    v: &'a V,
}

Since the detached guard does not own the data, it's inherently unsafe, but from my understanding and from asking others, it is still sound as long as the data is still coupled with the guard. Ref/Entry APIs still hold the guard, which upholds this invariant.

As a by-product of using this in mapref::multiple, I noticed that the RwLock guard is marked as GuardNotSend, meanwhile every iterator/ref is manually marked as Send/Sync. This seemed redundant, so I've additionally made the guard Send and then removed all manual send/sync impls.

[xacrimon]

lgtm