Faster time attributes

We spend a huge amount of cputime deserializing these fields:

```
                                  |    58  |   class Time < AttributeType
                                  |    59  |     def parse(input)
    1    (0.1%) /     1   (0.1%)  |    60  |       return nil if input.blank?
                                  |    61  |       case input
                                  |    62  |         when ::Time   then input
   61    (3.4%)                   |    63  |         when ::String then ::Time.parse(input)
                                  |    64  |         else input.to_time
                                  |    65  |       end
                                  |    66  |     end
                                  |    67  |     def encode(input) input ? input.utc.xmlschema : nil end
                                  |    68  |   end
```

/cc @github/perf