Fix invisibility check for some cases

[dvzubarev]

Old avy--visible-p couldn't handle properly such case: (get-char-property s 'invisible) returns t;
buffer-invisibility-spec is set to ((outline . t) t)

from buffer-invisibility-spec documentation:

The list can have two kinds of elements:
ATOM and (ATOM . ELLIPSIS).
A text character is invisible if its invisible property is ATOM 

Check for invisibility was taken from the Emacs function count-lines.

[aikrahguzar]

I was wondering about this: can't avy--visible-p be just be

(defun avy--visible-p (s)
  (not (invisible-p s)))

or are there cases which invisible-p doesn't handle. package-lint doesn't seem to think that using invisible-p requires raising the min Emacs version.