This PR adds partial supports for the uint64, string, and nvlist array types. Specifically, it adds NvEncode support, so they can be turned into NvLists. It was tested in conjunction with openzfs/zfs#12728, which adds const to a number of the array functions in the libnvpair code so that we don't need to use mutable references everywhere in rust.

Add a serde serializer that can serialize structs to NvList's.

Add a serde deserializer that can deserialize NvList's into structs.

Update to current state

When deserializing an nvlist, if the struct calls for an Option or a
newtype, expect simply the wrapped value in the nvlist (or absent from
the nvlist in the case of an Option).  This allows us to deserialize a
DATA_TYPE_UINT64 nvpair as a `struct NewType(u64)` or an `Option<u64>`.

The NvList type does not have interior mutability, so to modify it,
exclusive access is required (e.g. `NvList` or `&mut NvListRef`).
Therefore, a shared reference to NvList (i.e. `&NvListRef`) can be sent
to other threads, and accessed concurrently from multiple threads.  So
NvList can be marked `Sync`.

also fix some clippy warnings