Transparently support nested transactions in withTransaction

[rightfold]

This can be implemented using a WeakMap, mapping connections to nesting counts.

[akheron]

After #29 has been merged, this can be achieved by chaning the library monad to e.g.

newtype PG a = StateT (Maybe Int) (ExceptT PGError Aff) a

with the following rules for the state:

• Nothing -> not in a transaction
• Just 0 -> in a top-level transaction (BEGIN/ROLLBACK/COMMIT)
• Just n -> in n'th nested transaction (SAVEPOINT savepoint-n/ROLLBACK TO SAVEPOINT savepoint-n/RELEASE SAVEPOINT savepoint-n)

This change would also require us to add a function

runPG :: forall a. PG a -> Aff (Either PGError a)

as runExceptT alone wouldn't be enough anymore.