keys and vals support for sequences of map entries

[chrisrink10]

In #1011, the issue was raised that several core functions don't properly support nil values as they do in Clojure. Among those were keys and vals.

Upon further exploration, those functions support more than just mapping types and nil, but in fact any sequence yielding successive map entries (so calling (keys (seq {:a 1 ...})) will yield the keys of the sequence of the source map as well.