Spec out restricted profiles

[traverseda]

Right now the restricted profiles section of the spec is quite vague, and covers a lot of potential use cases. It's mentioned under "future discussions", well let's get that discussion started!

I believe that the only real way to implement such a broad goal would be to allow msgpack libraries to defer encoding/decoding of built-in msgpack objects to a user-supplied function, similar to how external types are handled now.

In my particular use-case I need msgpack to only encode immutable python types. That means I need to handle "map" types myself.

It might be simplest to have end users able to customize the mapping between language-types and msgpack types, overriding the libraries assumptions about type-coercion.