schema: complete validation

[kristoff-it]

In the construction of a Schema struct, we currently check for:

• Duplicate tag literal definitions
• Usage of unknown tag literals
• Duplicate struct definitions
• Duplicate struct field
• Usage of unknown struct names
• Duplicate mentions of the same Struct inside of a struct union expression

What's missing:

• Recognizing types of infinite size (ie self-references outside of a map or array container)
• Recognizing definitions impossible to reach from the root expression

Note that the presence of any automatically colors everything.

How to get started

Here we perform the aforementioned analysis: https://github.com/kristoff-it/ziggy/blob/4905290ae14391963ec2b11a72e81f779577824f/src/schema/Schema.zig#L168-L176

Below those lines it would make sense to call a new function that performs the remaining checks. It might make sense to add a field or two to the existing types (eg StructRule, Rule).