Split out fiber stacks from fibers.

This commit splits out a `FiberStack` from `Fiber`, allowing the instance
allocator trait to return `FiberStack` rather than raw stack pointers. This
keeps the stack creation mostly in `wasmtime_fiber`, but now the on-demand
instance allocator can make use of it.

The instance allocators no longer have to return a "not supported" error to
indicate that the store should allocate its own fiber stack.

This includes a bunch of cleanup in the instance allocator to scope stacks to
the new "async" feature in the runtime.

Closes #2708.

crates/runtime/src/lib.rs

@@ -38,7 +38,7 @@ pub use crate::export::*;
 pub use crate::externref::*;
 pub use crate::imports::Imports;
 pub use crate::instance::{
-    FiberStackError, InstanceAllocationRequest, InstanceAllocator, InstanceHandle, InstanceLimits,
+    InstanceAllocationRequest, InstanceAllocator, InstanceHandle, InstanceLimits,
     InstantiationError, LinkError, ModuleLimits, OnDemandInstanceAllocator,
     PoolingAllocationStrategy, PoolingInstanceAllocator, RuntimeInstance,
 };