add flag to mox to store execution trace, similar to cpu/memory profiling

useful for performance testing

profile.go

@@ -5,6 +5,7 @@ import (
 	"os"
 	"runtime"
 	"runtime/pprof"
+	"runtime/trace"
 )
 
 func memprofile(mempath string) {
@@ -43,3 +44,14 @@ func profile(cpupath, mempath string) func() {
 		memprofile(mempath)
 	}
 }
+
+func traceExecution(path string) func() {
+	f, err := os.Create(path)
+	xcheckf(err, "create trace file")
+	trace.Start(f)
+	return func() {
+		trace.Stop()
+		err := f.Close()
+		xcheckf(err, "close trace file")
+	}
+}