xsd:dateTime, xsd:time coercion is incorrect for fractional seconds

[mpoffald]

As predicted by this comment: #421 (comment), our strategy for converting decimal seconds to nanoseconds is incorrect. It only works with exactly 3 digits.

The following cases fail:

;;4 digits: errors
(datatype/coerce "11:14:32.8331Z" const/$xsd:time)
;;=>  error: java.lang.ArithmeticException: integer overflow
(datatype/coerce "2021-09-24T11:14:32.8331" const/$xsd:dateTime)
;;=>  error: java.lang.ArithmeticException: integer overflow

;;2 digits: values are incorrect
(datatype/coerce "11:14:32.83Z" const/$xsd:time)
;;=> #object[java.time.OffsetTime 0x7dcfbb4a "11:14:32.083Z"
(datatype/coerce "2021-09-24T11:14:32.83" const/$xsd:dateTime)
;;=>     #object[java.time.LocalDateTime 0x3260de9d "2021-09-24T11:14:32.083"]