Implementation of `Dict.drop`, `Dict.take`, and `Dict.split` (proposal #1069)

Methods in `Dict`, `ListDict`, and `HashDict`. Includes both doctests and unit tests.

Implementation of `drop` and `take` in `HashDict` achieved using `Dict` API rather than using low-level internal methods.  (I haven't delved into `HashDict`'s internals yet.)  This may be ripe for optimization, but now we have an initial working implementation with tests.