Rename pipe-buffered to pipe-series

The `pipe-buffered` function has been renamed to `pipe-series` to better
reflect its functionality. This change involves updating the function
name in the example file `examples/data_stream_pipe_buffered.myc` and
across the Rust source code in `src/ast/rtl/streams.rs`. Additionally,
test cases and documentation have been updated accordingly.

examples/data_stream_pipe_series.myc

@@ -0,0 +1,18 @@
+;; Skip: data output to stdout
+(do
+  (def EURUSD (create-m1-stream "EURUSD" (date "2020-01-03 09:30:00") (date "2020-01-03 10:00:00")))
+  (def GER40 (create-m1-stream "GER40" (date "2020-01-03 09:30:00") (date "2020-01-03 10:00:00")))
+
+  (def cnt 1)
+
+  (pipe-series 2 [EURUSD GER40]
+    (fn [eu ge]
+      (do
+        (def dax (.close (ge 0)))
+        (def eur (- dax (ge 1)))
+        (print cnt ": dax=" eur " (" (/ eur (.close (eu 0))) "$)" )
+        (assign cnt (+ cnt 1))
+      )
+    )
+  )
+)