Enum variant utils

Handy utilities for working with Rust enums.

• VariantDef - automatically define a const struct to make metadata about your enum variants available in a consistent way
• IsVariant - quickly check if something is an instance of a specific variant
• VariantName - quickly get (just) the variant name
• VariantPropsToJsonArray - get the variants properties as a Vec<serde_json::Value>

More tools and detailed docs to follow soon.