Refactor SMA implementation to lib/sma.myc

Move the SMA implementation from examples/err.myc and examples/sma.myc
to a new dedicated file in lib/. This change also cleans up unused code
and improves the SMA logic.

lib/sma.myc

@@ -0,0 +1,20 @@
+(do
+  (def SMA
+    (fn [length]
+      (do
+        (def history (series :float))
+        (def sum 0.0)
+
+		(fn [val]
+           (do
+             (def hist_len (len history))
+             (if (>= hist_len length)
+               (assign sum (- sum (history (- length 1)))))
+
+  		      (push history val)
+              (assign sum (+ sum val))
+
+              (if (>= hist_len length)
+                (/ sum length))
+           )))))
+)