Possibly remove `Dict\slice` and `Keyset\slice`

[fredemmott]

• Taking an arbitray slice of a keyedcontainer or set is conceptually weird.
• The most obvious cases are handled by Foo\chunk
• The other case is seeing it used to impose a maximum size on the container. In some cases, raising an exception for input that is too large may be appropriate, but potentially we may want to add Foo\take instead