Make LocalFuture Available to User Code

[ZwFink]

LocalFutures are lightweight future objects used internally within the runtime system for signaling within a PE. I'm working on an application that uses futures that are guaranteed to send results within the same PE. Because LocalFutures avoid communication, they should be made available to user code, with documentation making it clear that:

1. They cannot be used across PEs.
2. Values sent through a local future are passed by reference.