Refactor enums to address large_enum_variant warnings

[tizoc]

The codebase contains a bunch of enums that trigger large_enum_variant Clippy warnings.

Task

• Run cargo clippy -- -A clippy::all -W clippy::large_enum_variant to identify such enums.
• Use Box to minimize variant sizes where applicable.

[tizoc]

This may be covered already by #302