Fix: Improve series type handling and coercions

This commit enhances the series type handling in `src/ast/rtl/series.rs`
by introducing helper methods `as_float` and `as_int` in
`src/ast/types.rs` to manage type conversions more robustly.

It also updates the function overload resolution in `src/ast/types.rs`
to prioritize exact parameter matches before falling back to implicit
coercions.

Finally, the example `err.myc` has been updated to reflect a more
accurate implementation of a Simple Moving Average (SMA) calculation.

examples/sma.myc

@@ -1,25 +1,24 @@
 (do
-  (def SMA 
-    (fn [length] 
-      (do
-        (def history (series :float))
-        (push history 0)
-
-		(fn [val] 
-           (do 
-              (def sum (+ val (history 0)))
-			  (push history sum)
-              (/ sum length)
-(history 0)
-           ))
-      )))
-
-  (def sma20 (SMA 20))
-
-  (def n 100)
-  (while (> n 0) 
-    (do 
-      (print "val=" n " sma20=" (sma20 n))
-      (assign n (- n 1))
-    ))
+  (def SMA 
+    (fn [length] 
+      (do
+        (def history (series :float))
+        (push history 0)
+
+		(fn [val] 
+           (do 
+              (def sum (+ val (history 0)))
+			  (push history sum)
+              (/ sum length)
+           ))
+      )))
+
+  (def sma20 (SMA 20))
+
+  (def n 100)
+  (while (> n 0) 
+    (do 
+      (print "val=" n " sma20=" (sma20 n))
+      (assign n (- n 1))
+    ))
 )