feat: Add schema + code generation

[jbelkins]

Description of changes

• Adds the Swift Schema type, and related types for representing the Smithy model such as ShapeID and Node.
• Adds schemas for all Smithy primitive / "prelude" types.
• Adds code generation of schemas for all modeled shapes that would require one.

To illustrate, here's the generated schema for com.amazonaws.arcregionswitch#Approval, a two-case Smithy enum:

var schema__com_amazonaws_arcregionswitch__Approval: Smithy.Schema {
    .init(
        id: .init("com.amazonaws.arcregionswitch", "Approval"),
        type: .enum,
        members: [
            .init(
                id: .init("com.amazonaws.arcregionswitch", "Approval", "APPROVE"),
                type: .member,
                traits: [
                    .init("smithy.api", "enumValue"): "approve",
                ],
                target: Smithy.Prelude.unitSchema,
                index: 0
            ),
            .init(
                id: .init("com.amazonaws.arcregionswitch", "Approval", "DECLINE"),
                type: .member,
                traits: [
                    .init("smithy.api", "enumValue"): "decline",
                ],
                target: Smithy.Prelude.unitSchema,
                index: 1
            ),
        ]
    )
}

Scope

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jbelkins]

The Shape.schemaVar() method generates the Swift variable name for referencing a schema.

Depending on the namespace of the shape being referenced, it may either resolve to a code-generated schema or to a schema contained in the prelude.

The prelude schemas are in the Smithy runtime module and are defined below in Prelude.swift.

[jbelkins]

Members are never generated into their own variable; they are only generated inline into another schema's members array, hence this assertion.

[jbelkins]

Below is the list of traits that are included in the schema. There is a list in the spec of all the ones that should eventually be included, but for now I'm including only the most basic ones.

[jbelkins]

unused property

[jbelkins]

This type holds the actual content of a trait.

[jbelkins]

Added a test target to the Smithy target since none previously existed.

[jbelkins]

Fixes a warning about an unmodified var.

[jbelkins]

This function (and the private function below it) gets all the shapes needing schemas: inputs, outputs, errors, and their nested descendants, excluding schemas that are already provided in the Smithy prelude.

[jbelkins]

Like Models, schemas may either be rendered into individual files or one big Schemas.swift file.

[jbelkins]

Simple test on ShapeID to ensure it renders to String properly.

[jbelkins]

This type code-generates a schema for a Smithy model shape. A Schema is just a Swift-native type that holds info about a Smithy shape that is needed for serialization.

A couple notes:

• Schemas are generated as computed, global variables, with a naming scheme that ensures each schema has a unique name. Computed properties and reference types are used to avoid circular reference compile errors on self-referencing Smithy shapes.
• Schema and related types are being marked Sendable so that they can be assigned to global vars, but I don't expect them to have any sort of async access or functions.