Be a bit safer with reference counting.

We might want to revisit some uses of refcounts in the
ev module to be more efficient if we care about signal atomicity
(where memory order isn't really important) or multithreading atomicity.

src/core/capi.c

@@ -505,7 +505,7 @@ JanetAtomicInt janet_atomic_dec(JanetAtomicInt volatile *x) {
 #ifdef JANET_WINDOWS
     return InterlockedDecrement(x);
 #else
-    return __atomic_add_fetch(x, -1, __ATOMIC_RELAXED);
+    return __atomic_add_fetch(x, -1, __ATOMIC_ACQ_REL);
 #endif
 }