Document date/time/timestamp literal units and move to int64/micros since epoch/midnight (#97)
Document date/time/timestamp literal units and move to int64/micros since epoch/midnight (#97)

* Use int64 for encoding date literals as millis since epoch
* Move all temporal literals to int64 and document
substrait-io/substrait,substrait-io/substrait,substrait-io/substrait,substrait-io/substrait