Allow importing and resolving of chain specific types

[jsdw]

After #1 merges, we'll have the foundations in place, which is to say, we'll have a TypeRegistry that we can insert named types into and resolve them, possible in the scope of a specific pallet.

The next step, I think, should be to allow ingestion of some JSON file containing all of the necessary chain specific type information. So soemthing like:

• Define a struct eg TypeDescriptions which accepts all of the relevant type information for a specific chain, eg:
  • A list of types which are globally relevant (Name -> Shape).
  • A list of types scoped to specific pallets (pallet, Name -> Shape).
  • Any of the above which are relevant only between certain spec versions.
  • (There is a typesSpec thing too in PJS but need to ask Tarik about this and work out if it's needed)
• Have this format be deserializable from JSON. See about keeping the format similar to PJS if possible.
• Allow default "substrate" types to exist and then be extended by such a JSON document. This means either:
  • Allow the TypeDescriptions type to be built/extended from more than one such JSON document
  • Allow the resulting type registry (below, TypeRegistrySet) to be built from multiple such documents.
• Define something like a TypeRegistrySet which can be created by taking TypeDescriptions+ a spec version. This is internally a list of TypeRegistrys wherein we will try one at a time to locate some type. TypeRegistrySet will impl TypeResolver.
• Start cobbling together a JSON doc with basic Substrate/Polkadot types, and write an example which takes some bytes of such types and decodes them (being able to somewhat manually decode a historic block would be ideal).

[jsdw]

Closed by #3