Shift to use ordered maps for Schema.Record and Schema.Enumeration

[jdegoes]

Currently, we are using a Map for the keys in records and enumerations, which means that we cannot rely on the key ordering. Yet for some protocols, such as grpc, the ordering is very important: it must be stable and ideally reflected by case class / subtype ordering. In order to accommodate this, we need to switch to a map type that allows linear ordering of keys.

Scala has SortedMap, which could be used, but is not really ideal (it wants to order based on a property of the keys, but that would force us to store (Int, String) in the map, which is unpleasant to work with). Possibly we could use an ordinary map, together with an array that maintains key ordering; or we could create our own OrderedMap type.

[V-Lamp]

Hey, you could use ListMap

List map iterators and traversal methods visit key-value pairs in the order they were first inserted.