Relaxed (no Send + Sync) requirements for WasmerEnv for single-threaded applications

[4LT]

Motivation

I'm writing a program to run routines from 1 or more modules sequentially. While I have what I need to do this, it's a bit awkward using Arc<T> and Arc<Mutex<T>> simply because it's required for fields of types deriving from WasmerEnv that I want to share between host and modules.

Proposed solution

Either an alternative trait to WasmerEnv that doesn't derive from Send/Sync, or some configuration or switch that disables the requirement on build. There might be a "smarter" solution wherein a type could derive Send and Sync explicitly in addition to WasmerEnv and be used in multi-threaded contexts; or not derive Send and Sync and only be used in single-threaded contexts. In general, I think that would be preferable, however it would make the change a breaking change; besides, I don't even know if that would be possible.

Additional context

I understand that this is probably a low-reward change with a potentially high cost (I'm guessing this would require changes throughout the Wasmer project). It is perhaps just an aesthetic concern -- there might be a performance gain, but I don't have any benches to suggest it would be significant -- but I think that using Rc<T> and Rc<RefCell<T>> would make the code clearer to any potential consumers of my code that it's intended for single-threaded use only.

[kwonoj]

I think it's related to this issue, raised q at #2830, while in my case I can't just change Rc<RefCell> and curious if there's way to workaround this constraint.