chore: Convert Schema to class (2nd attempt)

[jbelkins]

Description of changes

In the original PR #1082, schemas were converted from value types that are created on demand using computed properties, to lazy globals that are created and retained on first use, then reused.

However, the change started causing compiler OOMs in the build system. That change was reverted in #1104 so that we could determine the cause of the OOMs and try the feature over again.

The cause was found to be that: member schemas were previously declared inline in the members array property of the parent schema. Sometimes (i.e. in the case of enums with hundreds of cases), this resulted in code-generated Swift literal expressions that were thousands of lines long. When these expressions were used in computed properties, they were handled by the compiler without problems.

However, when we instead tried to use these same large literal expressions as the RHS of an assignment, they started to consume a lot of time and RAM at type checking time. It has been noted before that very large arrays of complex literal expressions (i.e. structures like our schemas) cause high times & memory usage to type-check. See: swiftlang/swift#60549

Member schemas are now declared as their own separate schemas, assigned to a variable in the global namespace, and included by reference in the parent schema's members instead of being declared inline. This breaks up large composite schemas into much smaller expressions that are type-checked much easier.

Another change from the first attempt is that the schema global vars have explicit typing of Swift.Schema. Not providing explicit type info causes the compiler to incorrectly detect a circular reference when schema targets form a reference cycle. This discovery allows us to avoid the extra code-generated getters that we were using before, reducing the volume of code-generated expressions. (In reality, the circular reference chain is broken by the use of @autoclosure on the schema's target property, so the compiler is correct to not flag the reference.)

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]

it was an oversight for this property to be mutable, changed to let