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 @+ @%`).
Yeah, I can change that. the reason I put this here is actually because that's what I got when I did `:let @a` in Vim. Do you still want me to change it?