Address large_enum_variant exempt

As part of updating the clippy version, the `StoredValue` memory layout triggered the `large_enum_variant` warning in clippy. For now, this enum is tagged with `#[allow(clippy::large_enum_variant)]`. We can move each of the variants into `Box`es, or treat only the biggest variants, and keep the smallest and most used unchanged.

We should also instrument this to make sure we aren’t impacting perf since this is sensitive CC (@dwerner @xcthulhu)

Reference: https://github.com/casper-network/casper-node/blob/eec5cb5835966d0c58d1536934c95de59d2ab890/types/src/stored_value.rs#L37-L40