Changed rxt.cast to be useful for single values as well as objects/arrays.

Basically, I had situations where I needed to ensure a single variable was reactive, so I could reliably call .get() to get the value. This change makes the following possible:

    v = rxt.cast x                            # v will be a ObsCell
    v = rxt.cast y, "array"                   # v will be an ObsArray
    v = rxt.cast o, {x: "cell", y: "array"}   # Old behaviour