Uniform extract value from monad context.

I have discarded `run` function name  because it should be variadic and the name is semantically wrong for "extract" value from context. Making it multiarity it does that some monads like state, will have run/0 (arity 0)  with not implemented exception (because state monad for run needs a seed...) is not convinced me.

The new name `get-value` it's a good companion of `get-context` and it only extracts a value, do not  run the monad.