Disable internal functions once they're merged into core.

[joemcgill]

We need to make sure there are no errors for sites that are running our plugin once some of our internal functions are merged into core. To do so, we should depreciate our internal functions and make them wrappers for their counterparts in core when possible to be mindful of back compatibility for sites using our internal functions to build their own markup patterns.

[joemcgill]

@tevko and I discussed this, and the best idea would probably be to move all our internal functions that are now duplicated in core to a separate file that's only loaded if the functions don't already exist, i.e., the site isn't running a version of WP that already includes our functionality.