[DX] Simplify getting and setting of values without using the 'und' data structure in entities

One of the things I really hated about the change from D6 to D7 was the increase in complexity of the data structures in core. In particular, sites that did not need translation now had to deal with this extra und key everywhere.

There are many ways we can simplify the data structure in Backdrop without limiting functionality. Let's use this issue to talk about some of those options, and make a plan for Backdrop 2.