Serial events (#337)

* added "serial->on data received"

* updated info

* updated docs

* added readline to read entire buffer

* lazy initialize of serial buffers

* init async on event

* updated docs

libs/core/serial.ts

@@ -38,20 +38,20 @@ namespace serial {
     }
 
     /**
-    * Registers an event to be fired when a line has been received
-    */
-    // help=serial/on-line-received
-    // blockId=serial_on_line_received block="serial on line received"
-    // weight=21 blockGap=8
-    export function onLineReceived(body: Action): void {
-       // serial.onDataReceived("\n", body);
+     * Reads a line of text from the serial port.
+     */
+    //% help=serial/read-line
+    //% blockId=serial_read_line block="serial|read line"
+    //% weight=20 blockGap=8
+    export function readLine(): string {
+        return serial.readUntil(delimiters(Delimiters.NewLine));
     }
 
     /**
      * Returns the delimiter corresponding string
      */
     //% blockId="serial_delimiter_conv" block="%del"
-    //% weight=1
+    //% weight=1 blockHidden=true
     export function delimiters(del: Delimiters): string {
         // even though it might not look like, this is more
         // (memory) efficient than the C++ implementation, because the