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.

src/ast/compiler/type_checker.rs

@@ -422,9 +422,11 @@ impl TypeChecker {
                 let mut arg_types = Vec::with_capacity(inputs.len());
                 for input in inputs {
                     let typed_input = self.check_node(input, ctx, diag);
-                    // Unwrap Series(T) to T for the lambda argument
+                    // Unwrap Series(T) or Stream(T) to T for the lambda argument
                     let arg_ty = if let StaticType::Series(inner) = &typed_input.ty {
                         *inner.clone()
+                    } else if let StaticType::Stream(inner) = &typed_input.ty {
+                        *inner.clone()
                     } else {
                         StaticType::Any
                     };