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

@@ -5,7 +5,7 @@
 
   (def cnt 1)
 
-  (pipe-buffered 2 [EURUSD GER40]
+  (pipe-series 2 [EURUSD GER40]
     (fn [eu ge]
       (do
         (def dax (.close (ge 0)))