Add PlutusTx numeric instances for Term s PInteger

[blamario]

It's not the first time I get

test/Spec.hs:283:85: error:
    * No instance for (AdditiveSemigroup (Term s (PInteger @{S})))
        arising from a use of `+'

because I imported the wrong Prelude. We already have instance Num (Term s PInteger), we should add the PlutusTx.Prelude class instances as well. It literally doesn't matter which (+) is in scope, the result will be exactly the same.

[TotallyNotChase]

Relevant PR: #160
Relevant issue: #139

[L-as]

I don't think we should try to support PlutusTx more by bloating every Num etc. instance