#271 Disallow variable directory collisions

[bsutton]

Summary

• reject variable names that are also implicit variable directories
• validate multi-variable moves against the final namespace before mutation
• preserve atomic behavior when a namespace collision is rejected

Validation

• cargo fmt --all -- --check
• cargo clippy --locked -p revault_lockbox_api@0.0.4 --all-targets -- -D warnings
• cargo test --locked -p revault_lockbox_api@0.0.4

Closes #271