Support ?Send function environments

[Twey]

This PR is currently a draft, as the work needs to be ported to the other (non-sys) backends and also needs documentation in some places (not to mention some cleanups :)), but contains all public API changes and works for sys so I'm submitting it early for directional feedback.

Description

Apropos of #4634, parameterize the Store on a type of type-erased Objects. This enables building stores where the internal Object is Box<dyn Any> (instead of Box<dyn Any + Send>) to permit the storage of non-Send objects in the store.

Type defaults have been used to maintain API compatibility for the most part. However, this is still technically a breaking change, since it changes some public but less-used APIs in incompatible ways (e.g. adding type Object to AsStoreRef).

This PR focuses on function environments, as that's what we need to be non-Send, but could also be generalized to other parts of the API such as extern refs or trap handlers.

A trait Upcast<T> is introduced to wasmer-types for types that can type-erase T, and implemented for Box<dyn Any> and Box<dyn Any + Send>.

[syrusakbary]

Thanks for creating the PR, we believe is a step in the right direction. We need a bit more time to review and test in other places (like browser)

[Twey]

Appreciate it! Currently only the sys backend is done (hence the draft) but if you approve of the direction I'll get porting the other backends.