dateime and timestamp

[mtoy-googly-moogly]

TImes got left in a weird place when the timezone work was finished because there wasn't good clarity on the right thing. Now there is clarity.

Make the malloy gestures map as minimally and as naturally as possible onto the gestures prvodied by the database, instead of trying to create a "meta model" of time, just make the malloy gestures be better syntax for what is there.

Plus add support for the "civil" or "non time zone" time type, which is DATETIME in BigQuery.