Add Clone impls for NestedList and NestedDict

Now requires the syn/clone-impls feature (which is on by default).

Added

• Add Clone impl to parse::Braced wrapper (llwxntnt)
• Derive Clone impls for NestedList and NestedDict (nsysprpp)

Changed

• Require syn/clone-impls feature, needed for Clone impls for NestedList (among others) (qozqwmmk)
  • This is turned on in the default syn features, so shouldn't add any compile time in the common case.