new Ledger module; lens upgrade; cleanup

[sirlensalot]

Note: this gets us into a good place for pure operations (no primitive Seq ops outside of Ledger.hs; LogIndex used for all indexing so we're no longer tied to Int), but we'll still need to centralize log modification and access when we want to go with a virtual-memory structure -- our pure approach of grabbing the entire ledger (e.g. AppendEntries.hs) and calling pure functions on it won't cut it.

[buckie]

Main work needed is to change Ledger to Log. If you want to take a swing at fixing lastLogInfo this is a good time to.