coroutines: use photon work_pool when nr_jobs > 0, and use photon libc fn wrappers (#19711)

examples/coroutines/coroutines_bench.v

@@ -0,0 +1,37 @@
+// Build with (-gc none, until GC bug is fixed)
+// v -gc none -use-coroutines coroutine_benchs.v
+//
+import coroutines
+import time
+import net.http
+import sync
+
+const run_time = 10 * time.second
+
+fn request(mut mu sync.Mutex, count &int) {
+	for {
+		http.get('http://vlang.io/utc_now') or { panic(err) }
+		mu.@lock()
+		unsafe {
+			(*count)++
+		}
+		mu.unlock()
+	}
+}
+
+fn main() {
+	mut mu := sync.new_mutex()
+	mut count := 0
+
+	for _ in 0 .. 8 {
+		go request(mut mu, &count)
+	}
+	$if is_coroutine ? {
+		println('IS COROUTINE=true')
+		coroutines.sleep(run_time)
+	} $else {
+		println('IS COROUTINE=false')
+		time.sleep(run_time)
+	}
+	println('${count} requests made.')
+}