usb/cdc: Better ring buffer implementation (#5209)

* begin adding ring512 implementation

* refactor to make operation driven fuzz test

* refactor USBCDC.Read to use ring512

* try txhandler separate to flush

* working USBCDC with large packets

* fix binary size

* remove comment

* documentation improvements

src/examples/serial-stress/main.go

@@ -0,0 +1,29 @@
+package main
+
+import (
+	"machine"
+	"strconv"
+	"time"
+)
+
+func main() {
+	time.Sleep(2 * time.Second) // connect via serial
+	buf1 := makeBuffer('|', 600)
+	buf2 := makeBuffer('/', 600)
+	println("start")
+	serialWrite(buf1)
+	serialWrite(buf2)
+}
+
+func makeBuffer(sep byte, size int) []byte {
+	buf := make([]byte, size)
+	for i := 0; i < size-5; i += 5 {
+		buf[i] = sep
+		strconv.AppendInt(buf[i+1:i+1:i+5], int64(i), 10)
+	}
+	return buf
+}
+
+func serialWrite(b []byte) {
+	machine.Serial.Write(b)
+}