Find source locations even when clojure doesn't report them.

Search on emacs side when locations are not reported by clojure (pretty much for all interactive evaluations). Currently the regexp searches for "(def.*"  and works as expected for most common top level declarations.  

For inner declarations (letfn) a more extensive patch would be needed because the stored var for function `ns/foo/fn` is `ns/foo` and I don't see how to get info for inner function with existing middleware. 

This patch addresses #631.
I have changed the function to accept one argument (POS) which could be either a number-or-marker or a cons (LINE . COLUMN), thus retaining both the flexibility and the simplicity of the API.