Optimize core book module via book_cache port

I've work a lot with the book module since D5 and have many book related projects. In doing some profiling of said projects, I came across a few obnoxiously bloated functions in the book module which could serve cached output (basically the prev, up, next links are killer). Project page says it all as to why: https://www.drupal.org/project/book_cache

module in D7 adds: - a cache bin - hijacks a template preprocess functions to use book_cache's instead of core - triggers cache bin to clear at logical points in time (node save, insert, delete w/ ->book property set, etc) - has admin_menu support to clear via that UI if requested

It being so late int he D7 cycle I probably won't even bother trying to submit a patch for core for it but if this would be something that'd receive eyes / testing / consideration for inclusion in backdrop I'll throw it directly into Backdrop's book module and issue a PR.