A timestamp object

Features

• serde Serialization/deserialization with serde

• chrono conversion between chrono types, also required to parse/deserialize strings

• sqlx Encoding/Decoding for sqlx: As integer (nanoseconds) for Sqlite, as TIMESTAMP/TIMESTAMPTZ for PostgreSQL

• as-float-secs a legacy feature: the default de/serialization and string parsing is to/from float seconds (including integers)