ir: use Serializer.serialize where possible (#1809)

* ir: use Serializer.serialize where possible

* ir.Serializer: serialize MultiInfo correctly

* ir.Serializer: empty body needs to result in a skip statement

The firrtl grammar requires at least one statement.

* ir.Serializer: correctly serialize fixed point types and literals

* Serializer: change Seq -> Iterable in order to be Scala 2.13 compatible