Add epoch-based interruption for cooperative async timeslicing.

This PR introduces a new way of performing cooperative timeslicing that
is intended to replace the "fuel" mechanism. The tradeoff is that this
mechanism interrupts with less precision: not at deterministic points
where fuel runs out, but rather when the Engine enters a new epoch. The
generated code instrumentation is substantially faster, however, because
it does not need to do as much work as when tracking fuel; it only loads
the global "epoch counter" and does a compare-and-branch at backedges
and function prologues.

This change has been measured as ~twice as fast as fuel-based
timeslicing for some workloads, especially control-flow-intensive
workloads such as the SpiderMonkey JS interpreter on Wasm/WASI.

The intended interface is that the embedder of the `Engine` performs an
`engine.increment_epoch()` call periodically, e.g. once per millisecond.
An async invocation of a Wasm guest on a `Store` can specify a number of
epoch-ticks that are allowed before an async yield back to the
executor's event loop. (The initial amount and automatic "refills" are
configured on the `Store`, just as for fuel.) This call does only
signal-safe work (it increments an `AtomicU64`) so could be invoked from
a periodic signal, or from a thread that wakes up once per period.

crates/runtime/src/instance.rs

@@ -21,6 +21,7 @@ use std::convert::TryFrom;
 use std::hash::Hash;
 use std::ops::Range;
 use std::ptr::NonNull;
+use std::sync::atomic::AtomicU64;
 use std::sync::Arc;
 use std::{mem, ptr, slice};
 use wasmtime_environ::{
@@ -203,6 +204,11 @@ impl Instance {
         unsafe { self.vmctx_plus_offset(self.offsets.vmctx_interrupts()) }
     }
 
+    /// Return a pointer to the global epoch counter used by this instance.
+    pub fn epoch_ptr(&self) -> *mut *const AtomicU64 {
+        unsafe { self.vmctx_plus_offset(self.offsets.vmctx_epoch_ptr()) }
+    }
+
     /// Return a pointer to the `VMExternRefActivationsTable`.
     pub fn externref_activations_table(&self) -> *mut *mut VMExternRefActivationsTable {
         unsafe { self.vmctx_plus_offset(self.offsets.vmctx_externref_activations_table()) }