working on snippets

[default-kramer]

TODO

• Save+load snippets as files
• Allow previewing the snippet in isolation
• Why even copy the blockdata to a Snippet when you have an immutable stage?
• Is it a mistake to assume a sampler will be defined for every XZ in its bounds? Maybe we just need to use nullability...
• PutSnippetMutation is really a more general CopyBlocksMutation
• View layer by layer