Allow destructors to run in foreign threads. Fixes #69.

This change modifies destructors to anticipate that they can be called form a foreign thread if the GC is involved. The actual release of the reference will then happen deferred in the original thread.

source/vibe/core/internal/release.d

@@ -0,0 +1,17 @@
+module vibe.core.internal.release;
+
+import eventcore.core;
+
+/// Release a handle in a thread-safe way
+void releaseHandle(string subsys, H)(H handle, shared(NativeEventDriver) drv)
+{
+	if (drv is (() @trusted => cast(shared)eventDriver)()) {
+		__traits(getMember, eventDriver, subsys).releaseRef(handle);
+	} else {
+		// in case the destructor was called from a foreign thread,
+		// perform the release in the owner thread
+		drv.core.runInOwnerThread((h) {
+			__traits(getMember, eventDriver, subsys).releaseRef(cast(H)h);
+		}, cast(size_t)handle);
+	}
+}