Add Stream support for field accessors

The type checker now correctly handles unwrapping `Stream(T)` types for
lambda arguments.
A new `build_map_stream` function has been added to `rtl/streams.rs` to
facilitate field access on streams.
The `create-random-ohlc` function now returns a `Stream` instead of a
`Series`.
Examples have been updated to reflect these changes and demonstrate
stream usage.

examples/sma.myc

@@ -1,12 +1,10 @@
 #use rtl
 
 (do
-  (def sma20 (SMA 20))
-
-  (def n 100)
-  (while (> n 0)
-    (do
-      (print "val=" n " sma20=" (sma20 n))
-      (assign n (- n 1))
-    ))
+  (def src (create-random-ohlc 42 200))
+
+  (macro printx [s] `(fn [x] (print ~s x)))
+
+  (pipe [(.close src)] (printx "close=")) 
+  (pipe [(pipe [(.close src)] (SMA 20))] (printx "sma="))
 )

examples/streams.myc

@@ -0,0 +1,10 @@
+(do
+(def stream (create-random-ohlc 42 200))
+
+(macro prt [m] `(pipe (~m stream) (fn [s] (print ~m "=" s))))
+
+(prt .open)
+(prt .high)
+(prt .low)
+(prt .close)
+)