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/instance/allocator/pooling/unix.rs

@@ -58,11 +58,13 @@ pub fn decommit_table_pages(addr: *mut u8, len: usize) -> Result<()> {
     decommit(addr, len, false)
 }
 
+#[cfg(feature = "async")]
 pub fn commit_stack_pages(_addr: *mut u8, _len: usize) -> Result<()> {
     // A no-op as stack pages remain READ|WRITE
     Ok(())
 }
 
+#[cfg(feature = "async")]
 pub fn decommit_stack_pages(addr: *mut u8, len: usize) -> Result<()> {
     decommit(addr, len, false)
 }