Slight refactor of v6.e postcircumfix[;]
Slight refactor of v6.e postcircumfix[;]

- better fast pathing of simple cases
- simplification of recursive cases
- deleting non-existing elements returns Nil
- add support for :!exists
- fix several decont issues

Instigated by seeing results of writing extensive tests.

tony-o/rakudo,labster/rakudo,rakudo/rakudo,salortiz/rakudo,labster/rakudo,salortiz/rakudo,MasterDuke17/rakudo,labster/rakudo,salortiz/rakudo,rakudo/rakudo,MasterDuke17/rakudo,rakudo/rakudo,salortiz/rakudo,MasterDuke17/rakudo,tbrowder/rakudo,tbrowder/rakudo,MasterDuke17/rakudo,rakudo/rakudo,tony-o/rakudo,tbrowder/rakudo,tony-o/rakudo,rakudo/rakudo,tbrowder/rakudo,tony-o/rakudo,labster/rakudo,MasterDuke17/rakudo,salortiz/rakudo,salortiz/rakudo,tbrowder/rakudo,tony-o/rakudo,labster/rakudo,MasterDuke17/rakudo,labster/rakudo,rakudo/rakudo,tony-o/rakudo,tbrowder/rakudo