Fix clojure-find-ns when preceded by other forms

[p4v4n]

Closes #656
Follow-up for #661

• Have initially tried using a regex and syntax table first to detect if namespace is inside metadata but it was too difficult.
• This change re-frames the problem a little. Instead of checking if the namespace is inside string,comment, metadata e.t.c it checks if the current ns form is top-level form.
• Copied most of the test cases from @vemv previous commit.

[bbatsov]

What's the difference with clojure-top-level-form-p? It's a bit confusing to have 2 very similarly named functions IMO.

[p4v4n]

clojure--is-top-level-form-p checks if the sexp following the point is a top level form.
clojure-top-level-form-p checks the type of enclosing top level form w.r.t current point.
ex:
|(ns foo) -> (clojure--is-top-level-form-p) returns true.
(|ns foo) -> (clojure--is-top-level-form-p) returns nil.
(comment |(ns foo)) -> (clojure--is-top-level-form-p) returns nil.
(comment (|ns foo)) -> (clojure-top-level-form-p "comment") returns true.

Added some unit tests now for clojure--is-top-level-form-p.

[vemv]

I'd suggest naming it clojure--looking-at-top-level-form

This would be consistent in name and semantics with looking-at, I believe.

[p4v4n]

Renamed the fn to clojure--looking-at-top-level-form.

[vemv]

Fantastic work, kudos!