Support printing and reading of Java 8 java.time.Instant

In Clojure 1.9 alpha, limited support for Java 8 java.time.Instant was added, namely by (conditionally) extending the Inst protocol to that type.

It would be useful to enhance support for java.time.Instant further by

• installing a print-method and print-dup for java.time.Instant

• providing a read-instant function for reading java.time.Instant

This functionality is already provided in Clojure 1.8 today for the types java.util.Date, java.util.Calendar, and java.sql.Timestamp; extending it to java.time.Instant would be very helpful in environments using Java 8.