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

docs/reference/serial/read-string.md

@@ -0,0 +1,27 @@
+# Serial Read String
+
+Read the buffered serial data as a string
+
+```sig
+serial.readString();
+```
+
+### Returns
+
+* a [string](/reference/types/string) containing input from the serial port. Empty if no data available.
+
+### Example
+
+The following program scrolls text on the screen as it arrives from serial.
+
+```blocks
+basic.forever(() => {
+    basic.showString(serial.readString());
+});
+```
+
+### See also
+
+[serial](/device/serial),
+[serial write line](/reference/serial/write-line),
+[serial write value](/reference/serial/write-value)