Consider renaming or aliasing Observable as Subject

Definitions of the "Observer Pattern" use the term `Subject` to refer to what Hyperstack calls `Observable`.

Easy enough to make `Hyperstack::State::Subject` a synonym and deprecate Observable.

Note that the module `Hyperstack::State::Observer` already uses the correct term.

For example:

```ruby
class MyObject
include Hyperstack::State::Subject # vs. Hyperstack::State::Observable
...
end
```