Add ivy-state struct

Add ivy-state struct

* ivy.el (ivy-state): New defstruct.
(ivy-last): A single global to store an `ivy-state' struct.
(ivy-require-match): Move into `ivy-last'.
(ivy-done): Update.
(ivy-resume): New defun. Initial draft, kind of works for swiper. Need
to add a callback of what to do with the result as an argument.
(ivy-read): Store all arguments into `ivy-last'.
(ivy-completing-read): Update doc.