1.4.0

• Add Builder::set_repair_callback() which can be used to set a callback function that will be invoked if the database needs repair while opening it.
• Add support for custom storage backends. This is done by implementing the StorageBackend trait and
  using the Builder::create_with_backend function. This allows the database to be stored in a location other
  than the filesystem
• Implement RedbKey and RedbValue for char
• Implement RedbKey and RedbValue for bool
• Implement TableHandle for Table
• Implement MultimapTableHandle for MultimapTable
• Fix panic that could occur when inserting a large number of fixed width values into a table within a single transaction
• Fix panic when calling delete_table() on a table that is already open
• Improve performance for fixed width types
• Support additional platforms