Avoid unnecessary work in :alias and :refer candidate search

- The :alias expression no longer iterates through (all-ns) if no
  aliased symbols are present in the body

- The :refer expression no longer iterates through every var of every
  namespace; (ns-publics ns) returns a map keyed by symbols so a simple
  contains? will do the same work

While these optimizations offer minor performance gains, the gain in
code clarity also merits their inclusion.
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