This change makes a few simplifications to how properties are exposed in
the system, mostly in the name of usability, but also to feel a bit more
like "idiomatic JavaScript". Namely:
* Rename `then` to `mapValue`. This hopefully helps to suggest that this
is meant for a dataflow style of programming.
* Move Property<T> into the runtime module, and remove PropertyState<T>,
collapsing back down to a single type. This also eliminates some of the
messy internal runtime casting, accessing of internal members, etc.
* Export a Computed<T> interface from the root of the module. This is
the entirety of the public-facing surface area for properties, and
exposes that single `mapValue` member function. The internal runtime
logic understands how to handle Property<T>s specifically in addition
to Computed<T>s more generally (in case someone writes their own).
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