Use dependent types for PParams

Change PParams to use dependent types in the spirit of the Shelley spec. For this, we replace the record by:

```agda
data PParamName : Type
maxBlockSize : PParamName

typeOfPParam : PParamName -> Type

PParams : Type
PParams = (ppn : PParamName) -> typeOfPParam ppn

PParamsUpdate : Type
PParamsUpdate = (ppn : PParamName) -> Maybe (typeOfPParam ppn)

PParamGroup : PParamName -> PParamGroup
```

This solves some awkwardness, for example, in that the current specification doesn't formally link parameter groups to parameters.