optimisation: standard library seq functions should coerce inputs before using them

[ikappaki]

Extracted into its own issue from #1198 (comment) by @chrisrink10 :

The first issue is that seq library functions don't coerce their inputs into seqs before operating on them. Clojure does this in most of it's seq library:

(when-let [s (seq coll)]
  ;; ...
  )