Allows to convert into/from micros_since_epoch:u64 for TimeStamp

[mamcx]

Description of Changes

To make it possible to integrate spacetimedb::Timestamp with chrono, I add a way to convert into/from micros_since_epoch:u64

Expected complexity level and risk

1

[joshua-spacetime]

Any reason not to prefer From<Timestamp> for u64?

[mamcx]

Any reason not to prefer From<Timestamp> for u64?

Mainly for the semantics (so is clear wich unit is used here)

[gefjon]

Any reason not to prefer From<Timestamp> for u64?

Agreed w/ Mario; Timestamp::from(n) could do any number of different things, e.g. treat as seconds, millis, micros or nanos, and doesn't necessarily refer to an offset from the UNIX epoch (e.g. Rust's std::time::Instant is not), but Timestamp::from_micros_since_epoch says exactly what it does.