This commit implements an optimization to help improve concurrently
creating instances of a module on many threads simultaneously. One
bottleneck to this measured has been the reference count modification on
`Arc<HostFunc>`. Each host function stored within a `Linker<T>` is
wrapped in an `Arc<HostFunc>` structure, and when any of those host
functions are inserted into a store the reference count is incremented.
When the store is dropped the reference count is then decremented.

This ends up meaning that when a module imports N functions it ends up
doing 2N atomic modifications over the lifetime of the instance. For
embeddings where the `Linker<T>` is rarely modified but instances are
frequently created this can be a surprising bottleneck to creating many
instances.

A change implemented here is to optimize the instantiation process when
using an `InstancePre<T>`. An `InstancePre` serves as an opportunity to
take the list of items used to instantiate a module and wrap them all up
in an `Arc<[T]>`. Everything is going to get cloned into a `Store<T>`
anyway so to optimize this the `Arc<[T]>` is cloned at the top-level and
then nothing else is cloned internally. This continues to, however,
preserve a strong reference count for all contained items to prevent
them from being deallocated.

A new variant of `FuncKind` was added for host functions which is
effectively stored via `*mut HostFunc`. This variant is unsafe to create
and manage and has been documented internally.

Performance-wise the overall impact of this change is somewhat minor.
It's already a bit esoteric if this atomic increment and decrement are a
bottleneck due to the number of concurrent instances being created. In
my measurements I've seen that this can reduce instantiation time by up
to 10% for a module that imports two dozen functions. For larger modules
with more imports this is expected to have a larger win.