Time/Date types for use on the frontend. This provides the following
equivalents:

| #time/date "2020-10-10"                      | java.time.LocalDate     | goog.date.Date                 |
| #time/time "05:30:45"                        | java.time.LocalTime     | lambdaisland.deja-fu/LocalTime |
| #time/date-time "2020-10-07T12:16:41.761088" | java.time.LocalDateTime | goog.date.DateTime             |

For all of these we make sure the tagged reader literals are read correctly,
and that values of these types print with these reader tags.

All times are "local", they don't carry timezone information.

We also implement an ad-hoc API here, adding things as we need them, making
sure that these functions hide the details of these types. All date/time
manipulation on the frontend should go through this namespace. Avoid calling
properties/methods of concrete types directly, and avoid using js/Date.

Create a new local-date-time using the same syntax from goog.Date or js/Date:
  (local-date-time 2021 4 19 15 39 0 0)

Parse a time string:
  (parse-local-date-time "2021-01-01T23:59")

Get the current time:
  (local-date-time)

Adjust the seconds component:
  (assoc (local-date-time) :seconds 0)

Get the next day:
  (add-interval (local-date-time) {:hours 1})

Works well with threading:
  (-> (parse-local-date-time "2021-01-01T23:59")
   (assoc :seconds 0)
   (to-local-date))