Add a data structure for memory conf reading and writing (#1041)

* Copy MemConf.scala from ucb-bar/barstools#35 into memlib.
This provides a data structure wrapper around the existing memory conf format
which contains both reading and writing methods, making it easier to write code
that needs to read the format.

* Add MemConf tests and use a Map[MemPort, Int] for port lists instead of a Seq[MemPort] which is a bit less fragile.