Audit & document definitions of compute functions

[lwwmanning]

And document accordingly (related to #574)

A non-exhaustive list of potential inconsistencies:

• Should SliceFn::slice implementations for arrays do bounds checking, or always assume that it was done by the top-level compute fn?
• What should the behavior of SliceFn::slice be if start >= stop (Rust slice semantics would return empty, we are inconsistent)
• What is the behavior of fill_forward? Should the resulting Array now NonNullable, or should it have the exact same DType?
• etc.