Columnar serialization

[frankmcsherry]

Support for columnar::Stash backed serialization for the Updates type in the columnar module.

The commits tell the story, but it's about swapping Stash<Lists> in place for Lists, which is an enum that allows the typed Lists or [u8] backing that can be zero-copy decoded and used as a Lists::Borrowed<'_>. The work first introduces a bit of discipline around typed data vs views (borrowed data) vs stash-backed data. The final commit introduces the serialization itself.