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 @+ @%`).
@Kazark if it's a general "parse failed" message I'd prefer to use `Resources.Parse_Error`.  If you want to add a specific one here though I don't mind that at all.  It's actually preferable, I just often run out of creativity with error messages and go back to the general one.