Async counter part to `wasmtime::Linker::module` (`module_async`)

[CBenoit]

Feature

There is no Linker::module_async method to add a module to the linker in async context.
I suggest to add one or if there is a reason why it can't be done, I would like to know why.

Benefit

Ability to easily link modules together like in the example but in async context.

Implementation

I gave a try in my fork but it doesn't work apparently.

thread 'two_plugins' panicked at 'called `Result::unwrap()` on an `Err` value: couldn't instanciate `linking1`