ws2812: make AVR support more robust

* Merge AVR support for Digispark and non-Digispark
* Fix the error "inline assembly requires more registers than available"
* Use a single file for all AVR targets, in a similar style as the
  Xtensa support.

examples/ws2812/arduino.go

@@ -0,0 +1,9 @@
+// +build arduino
+
+package main
+
+import "machine"
+
+// Replace neo in the code below to match the pin
+// that you are using if different.
+var neo = machine.D2

examples/ws2812/others.go

@@ -1,4 +1,4 @@
-// +build !digispark
+// +build !digispark,!arduino
 
 package main