Introduce some new template functions for navigation:

 * `get_the_post_navigation()` and `the_post_navigation()` for navigation to the next and previous post.
 * `get_the_posts_navigation() and `the_posts_navigation()` for navigation to the next and previous page of posts.
 * `get_the_pagination()` and `the_pagination()` for paginated navigation between pages of posts. Uses `paginate_links()`.

This reduces the need for themes to define their own sets of navigation functions.

Fixes #29808.
Props obenland.
 


git-svn-id: https://develop.svn.wordpress.org/trunk@30065 602fd350-edb4-49c9-b593-d223f7449a82