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/environ/src/tunables.rs

@@ -36,6 +36,9 @@ pub struct Tunables {
     /// will be consumed every time a wasm instruction is executed.
     pub consume_fuel: bool,
 
+    /// Whether or not we use epoch-based interruption.
+    pub epoch_interruption: bool,
+
     /// Whether or not to treat the static memory bound as the maximum for unbounded heaps.
     pub static_memory_bound_is_maximum: bool,
 
@@ -88,6 +91,7 @@ impl Default for Tunables {
             parse_wasm_debuginfo: true,
             interruptable: false,
             consume_fuel: false,
+            epoch_interruption: false,
             static_memory_bound_is_maximum: false,
             guard_before_linear_memory: true,
             generate_address_map: true,