3.10.2: concat() and precat()

Release 3.10.2 2022-05-08: concat() and precat()

• Implemented concat() and precat() methods on ImMap, MutMap, ImSet, MutSet, ImRrbTree and MutRrbTree
  because:
  • concat() was implemented on PersistentVector() because it's a cheap operation.
  • concat() and precat() are cheap and sensible on the other collection types (RRB Tree and non-sorted maps and sets)
  • Now they all return their original collection type instead of an Xform.
  • Mirko Raner (@raner) brought this to my attention in #49 - Thank you @raner!
• Added a few @NotNull and @contract annotations where they were missing.