Non-ephemeral values analyzer

[bjorg]

For some types, it would be helpful to make sure that they are always used in some capacity. Task comes to mind when returned by an async method. Without using await, the called method will never run passed the first async operation. I use the term "non-ephemeral" to indicate that these values cannot be simply forgotten (i.e. they must be used at least once). "Linear" is a similar concept, but is further restricted by the notion that values must be used exactly once.