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/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)
+)