Update examples to use StreamNode and Series with lookback

The `PipelineNode` has been refactored into `StreamNode`. This commit
updates the example files to reflect this change and also updates the
`series` constructor to accept a lookback parameter.

The `PipelineNode` was an artifact of a previous implementation and is
no longer needed. The `StreamNode` is the appropriate type for
representing the output of a pipe operation.

The `series` function now requires a `lookback` argument to specify the
maximum number of items to store. This makes the behavior of series more
explicit and prevents accidental unbounded memory growth.

examples/sma.myc

@@ -1,12 +1,22 @@
 #use rtl
 
 (do
-  (def src (create-random-ohlc 42 200))
+  (def src (create-random-ohlc 42 20000))
+
+  (macro printx [s] `(fn [x] (print ~s x)))
+
+;  (pipe src print)
+;  (pipe [(pipe [(.close src)] (SMA 20))] (printx "sma20="))
+;  (pipe [(pipe [(.close src)] (SMA 5))] (printx "sma5="))
+;  (pipe [(pipe [(.close src)] (SMA 100))] (printx "sma100="))
 
-  (macro printx [s] `(fn [x] (print ~s x)))
+(macro cache [lookback type src] `(do (def data (series ~lookback ~type)) (pipe [~src] (fn [s] (do (push data s)))) data))
 
-  (pipe src print) 
-  (pipe [(pipe [(.close src)] (SMA 20))] (printx "sma20="))
-  (pipe [(pipe [(.close src)] (SMA 5))] (printx "sma5="))
-  (pipe [(pipe [(.close src)] (SMA 100))] (printx "sma100="))
-)
+
+
+;  (def bars (series 20 :float))
+;  (pipe [src] (fn [s] (do (push bars (.close s)))))
+
+(cache 20 :float src)
+  
+)