Remove Send bound from FunctionEnv

[Twey]

Description

Requiring that function environments be Send is quite onerous on the Web, in which many objects (including the Wasmer Instance itself, should we wish to pass around a reference to it) are not Send. The function environment is not passed between threads in Wasmer, and in typed usage the user never sees the trait object, only concrete instances of it after casting — which means that all user-visible types will remain Send if the user should wish to pass them around.

[Twey]

Hi @syrusakbary, the system automatically assigned you to review this — but if somebody else should do it please let me know whom to request 😃

[syrusakbary]

Hey @Twey , we want to set a sound type system for the Store, in a way that if the Store is Send+Sync then the functions have to be as well. But if is not... then they don't need to.

But we need to define a bit better this, that's why the review on this PR is a bit halted

[Twey]

Hey @syrusakbary, thanks for getting back to me! 😄

The easiest way I can think of to do that would be to remove the Box<dyn Any> from the VMFunctionEnvironment struct and make it a parameter instead (which could be instantiated to Box<dyn Any> or Box<dyn Any + Send> as the user likes — or kept as the concrete T that will be used at a higher level). That way both the store and the function environment will inherit their Sendness from the parameter. Do you think that's worth me making a PR for? Or is there a reason we can't have type parameters on VMFunctionEnvironment?