0.18.0

• Improve errors to be more granular. Error has been split into multiple different enums, which
  can all be implicitly converted back to Error for convenience
• Rename savepoint() to ephemeral_savepoint()
• Add support for persistent savepoints. These persist across database restarts and must be explicitly
  released
• Optimize restore_savepoint() to be ~30x faster
• Add experimental support for WASI. This requires nightly
• Implement RedbKey for ()
• Fix some rare crash and data corruption bugs