Improve macroexpansion facility.

Now the value 'tidy for cider-macroexpansion-suppress-namespaces (which
is the value "tidy" for the defn macroexpansion's suppress-namespaces
argument) tidys up the macro expansion:

  - Vars from the current namespace are printed with simple name
  - Vars from other namespaces which are refered are printed with their
    simple name (e.g., usually all of clojure.core)
  - Vars from other namespaces that have an alias are printed using that
  - Other vars are printed fully-qualified