`t` is not a valid keymap.  I looked through the other minor-mode
definitions and didn't see any other such uses.

Found that this was causing me problems, see Wilfred/helpful#114.

* lisp/pdf-cache.el (pdf-cache-prefetch-pages-function-default): Use
  stable pdf-util-remove-duplicates and adapt new pdf-info interface.

* lisp/pdf-cache.el (pdf-cache--prefetch-pages): Check we are still in
  the same buffer and the major-mode is right.
  (pdf-cache--prefetch-pages): Guard against multiple entrances.

* lisp/pdf-tools.el: Add let-alist as requirement.

* lisp/pdf-occur.el (pdf-occur-add-matches, pdf-occur-start-search): Use new interface.

* lisp/pdf-isearch.el (pdf-isearch-search-page): Use new interface.

* lisp/pdf-info.el (pdf-info-query--transform-action,
  pdf-info-query--transform-response): Prefer alists over trees or
  positional lists, where appropriate.

  (pdf-info-search-string,pdf-info-search-regexp,pdf-info-pagelinks
  pdf-info-charlayout,pdf-info-pagelinks, pdf-info-outline,
  pdf-info-synctex-forward-search, pdf-info-synctex-backward-search):
  Updated documentation.

* test/pdf-info.ert: Require let-alist, use new interface.

* Makefile (bytecompile, test): Use cask.

* Cask (depends-on): Add let-alist.

* lisp/pdf-outline.el: Use new interface.

* lisp/pdf-links.el: Use new interface.

* lisp/pdf-cache.el: Use new interface.

* lisp/pdf-sync.el: Use new interface.

* lisp/pdf-cache.el (pdf-cache-renderpage-text-regions): Use cached images with exact width only.
(pdf-cache-renderpage-highlight): Dito.

* lisp/pdf-info.el (pdf-info-textlayout): => pdf-info-textregions

* lisp/pdf-cache.el (textlayout): Adjust.

* lisp/pdf-view.el (pdf-view-text-regions-hotspots-function): Adjust.