core: add UNIT_GET_SLICE() helper

core: add UNIT_GET_SLICE() helper

In a later commit we intend to move the slice logic to use proper
dependencies instead of a "UnitRef" object. This preparatory commit
drops direct use of the slice UnitRef object for a static inline
function UNIT_GET_SLICE() that is both easier to grok, and allows us to
easily replace its internal implementation later on.