* lisp/isearch.el: Misc simplification; use defstruct.

* lisp/isearch.el: Misc simplification; use defstruct.
(isearch-mode-map): Dense maps now work like sparse ones.
(isearch--state): New defstruct.
(isearch-string-state, isearch-message-state, isearch-point-state)
(isearch-success-state, isearch-forward-state)
(isearch-other-end-state, isearch-word-state, isearch-error-state)
(isearch-wrapped-state, isearch-barrier-state)
(isearch-case-fold-search-state, isearch-pop-fun-state): Remove,
replaced by defstruct's accessors.
(isearch--set-state): Rename from isearch-top-state and change
calling convention.
(isearch-push-state): Use new isearch--get-state.
(isearch-toggle-word): Disable regexp when enabling word.
(isearch-message-prefix): Remove unused arg _c-q-hack.
(isearch-message-suffix): Remove unused arg _ellipsis.