time: clean up MarshalJSON + support xml #2793
Description
1. Format should produce years that are at least 4 digits.
2. The year-padding code in MarshalJSON should go away.
3. The bulk of the MarshalJSON function (all except the "s)
should be pulled into t.RFC3339().
4. MarshalJSON just calls RFC3339 and then adds quotes.
5. package xml should recognize time.Time and call RFC3339
to get a representation/call time.Parse(time.RFC3339) to
parse a representation.