Outline of structs as a general 'definition' system

[flyingsilverfin]

Usage and product changes

We introduce the architecture around structs definitions, and definitions in general.

The idea of a definition is that it is a general schema construct outside the regular thing and type system - this will initially be used for structs and functions.

Definitions are interesting because their interesting payload is in the value, not the key. However, the key contains the ID of the definition. As a result, you can consider the new DefinitionKey to be analogous to TypeVertex, but the interesting data structure is the StructDefinition, which serialises a struct's content.

In doing this, we also realise that ValueType must encompass the built-in value type category, plus the ID of the struct definition, and create a new ValueTypeCategory which can map to a Prefix and doesn't contain the ID of struct definitions. Lastly, we create a the serialised form of ValueTypeBytes, which is used as the property of an attribute type's value type, including the definition ID if it exists.

What isn't implemented yet:

1. The indexing + retrieval by name of value types through the TypeManager
2. Serialisation of StructDefinition
3. The data insertion and deletion pattern of struct instantiations

[vaticle-bot]

PR Review Checklist

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Trivial Change

• This change is trivial and does not require a code or architecture review.

Code

• Packages, classes, and methods have a single domain of responsibility.
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• The code is canonical and the minimum required to achieve the goal.
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Architecture

• Any required refactoring is completed, and the architecture does not introduce technical debt incidentally.
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