Common language - needed, provided, and allowed

Both Datums and Scripts have a similar set of relationships:
* items that are **needed**
* items that are **provided** by the transaction
* items that are **allowed**

Items which are allowed but not needed may not appear to have any use in the STS rules, though they can be very helpful inside of the transaction context used by plutus scripts.

Note that we always want `needed ⊆ provided ⊆ allowed`.

**Action items**:

Use these words consistently in both the spec and the implementation. We should also use these words wherever else is appropriate, such as tests, etc.