Set registers via :let

This has a number of interesting uses, such as `let @+=@%`, which copies the currently filename to the system clipboard rather succinctly. Notably, the display version of let does not work with register names (e.g.  `let @+ @%`).
Okay, yeah, I was just trying to emulate Vim more closely. However, this case is probably not worth emulating, because "invalid expression" is not a very helpful message in this case and may just be an accident of how Vim's parser works.