Needs Design: Allow &mut and raw pointer returning functions in interfaces

[karthikbhargavan]

We currently do not support code that returns &mut or *mut and more generally we ban these patterns from appearing in many places in the AST. However, it may be safe to model these types abstractly and allow them to appear in "interfaces", i.e. functions with empty bodies that are place-holders and will be either hand-modeled or abstracted for verification.

This needs a design document.