Version 0.3.0

Massive rework, with a focus on secondary maps and custom keys to prevent cross-slotmap key usage.
Removed DenseSlotMap in favour of HopSlotMap as the latter performs better when secondary maps are in use.
Unfortunately due to the redesign the first slot in a slot map must now always be empty. This means some deserializations of slot maps serialized with a version before 0.3.0 can fail.
Added SecondaryMap and SparseSecondaryMap, which allows you to associate extra data with keys given by a slot map.
Added DefaultKey, custom key types, and support for them on all slot maps and secondary maps. You must now always specify the key type you're using with a slot map, so SlotMap<i32> would be SlotMap<DefaultKey, i32>. It is recommended to make a custom key type with new_key_type! for any slot map you create, as this entirely prevents using the wrong key on the wrong slot map.
KeyData now has as_ffi and from_ffi functions that convert the data that makes up a key to/from an u64. This allows you to use slot map keys as opaque handles in FFI code.

Version 0.2.1

Fixed a potential uninitialized memory vulnerability. No uninitialized memory was read or used directly, but Rust's assumptions could lead to it. Yanked all previous versions as they were all vulnerable.
Made a Key member non-zero so that Option<Key> is optimized.

Start of version history.