Handle multiple clauses of the same type in ns-to-map

e.g.
        (ns my.ns
          (:require [foo :as f])
          (:require [bar :as b] :reload))
Extract parse-ns-map from asplode

The ns parser can now be used independently:

(parse-ns-map (ns-to-map '(ns my.ns …)))
Parse require flags as sets of ns-syms

This will allow the emission of separate :require clauses for different
sets of require flags.
Generate multiple require clauses for each set of require flags

https://github.com/technomancy/slamhound/pull/53#issuecomment-28137768