sidekick: a better representation for `Map<K, V>`

Maps are represented as a message with a key and value fields... This is a terrible representation because the codecs constantly find themselves asking "is this a map?" and "what is the key type?" and writing functions or loops to do so.

We should change maps to be top-level entities like `api.Enum` or `api.Message` with their own `Typez` and all.

Then we can simplify the code and (probably) enforce rules such as "a map cannot have a map as key or value".

Start by locating the existing map representation and the codecs that ask whether a value is a map or determine its key type. Compare that design with the top-level api.Enum and api.Message entities and their Typez handling. Done should mean maps have their own representation and Typez, codec logic is simplified, and the stated map key/value rules are enforced.

Written by the indexing model from the issue text.