Missing trait implementations

[Luro02]

There are some traits missing for StableVec that could be implemented.

Important traits:

• Hash
• Ord
• Write
• RefUnwindSafe
• UnwindSafe
• PartialOrd

serdes Serialize and Deserialize could also be supported?

Convenience:

• From<BTreeMap<usize, T>>
• Extend<&'a T>
• From<&'_ str>
• From<BinaryHeap<T>>
• From<Box<[T]>>
• From<CString>
• From<String>
• From<Cow<'a, [T]>>
• PartialEq<&'_ [B; N]>
• PartialEq<&'_ mut [B]>
• PartialEq<[B; N]>

[LukasKalbertodt]

This crate is deliberately low level; API convenience is not particularly high on the priority list. Of course, if adding an impl makes sense and has a low maintenance cost, we can add it.

Ord, PartialOrd

I think it's not completely obvious how those would be implemented. Just use the filled slots for comparison? This could easily lead to confusion and subtle bugs. I think I prefer not to implement those.

Hash

Since StableVec implements PartialEq, we need to uphold the property k1 == k2 -> hash(k1) == hash(k2). And our PartialEq impl currently does this: "empty slots, capacity, next_push_index and the indices of elements are all checked. In other words: all observable properties of the stable vectors need to be the same for them to be equal."

So implementing Hash might be non-trivial.

RefUnwindSafe and UnwindSafe

Those are in a strange place right now. I already read proposals to deprecated them.