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/misc/run-examples/src/main.rs

@@ -73,8 +73,8 @@ fn main() -> anyhow::Result<()> {
                 format!("examples/{}.{}", example, extension)
             };
 
-            if extension == &"cc" && !std::path::Path::new(&file).exists() {
-                // cc files are optional so we can skip them.
+            if !std::path::Path::new(&file).exists() {
+                // C and C++ files are optional so we can skip them.
                 continue;
             }