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/soa_series.myc

@@ -2,7 +2,7 @@
 ;; Benchmark: 1.4us
 ;; Benchmark-Repeat: 1406
 (do
-  (def my_ticks (series {:price :float :volume :int :msg :text}))
+  (def my_ticks (series 100 {:price :float :volume :int :msg :text}))
 
   (push my_ticks {:price 10.5 :volume 100 :msg "A"})
   (push my_ticks {:price 11.2 :volume 200 :msg "B"})