Introduce `get_previous_post_link()`, `get_next_post_link()`, and `get_adjacent_post_link()`. Allows developers to retrieve the value without `echo`ing.

Props yoavf, markjaquith, SergeyBiryukov.
Fixes #17302.



git-svn-id: https://develop.svn.wordpress.org/trunk@25327 602fd350-edb4-49c9-b593-d223f7449a82
Add filter `flush_rewrite_rules_hard` to give devs chance to prevent writing to file when hard-flushing rewrite rules.

Props jeremyfelt, nacin.
Fixes #23491.



git-svn-id: https://develop.svn.wordpress.org/trunk@25328 602fd350-edb4-49c9-b593-d223f7449a82