v0.3.0

• Added support for updating rows.
• Added Table::dummy method, which makes it easier to do partial updates.
• Reused table types in the generated API for both naming TableRow<User> and dummies User {name: "steve"}.
• Forbid Option in unique constraints.
• Renamed ThreadToken to LocalClient.
• Renamed and moved read and write_lock to transaction and transaction_mut.
• Check schema_version at the start of every transaction.
• Simplify migration and borrow LocalClient only once.
• Renamed Prepare to Config and simplified its API.