Fixnum implementations should not expose `slot_layout` functionality

[unzvfu]

From https://github.com/data61/cuda-fixnum/issues/18:

Algorithms that use a particular fixnum_impl should be expressed entirely in terms of the fixnum_impl interface; slot_layout operations can be exposed via that interface, but they should be given semantically significant names (e.g. shfl_up0 is a shift-left in (most? all?) classical fixnum representations).