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/libcalls.rs

@@ -557,3 +557,11 @@ pub unsafe extern "C" fn wasmtime_out_of_gas(vmctx: *mut VMContext) {
         Err(err) => crate::traphandlers::raise_user_trap(err),
     }
 }
+
+/// Hook for when an instance observes that the epoch has changed.
+pub unsafe extern "C" fn wasmtime_new_epoch(vmctx: *mut VMContext) -> u64 {
+    match (*(*vmctx).instance().store()).new_epoch() {
+        Ok(new_deadline) => new_deadline,
+        Err(err) => crate::traphandlers::raise_user_trap(err),
+    }
+}