Fix Vec2D related brokenness

I'm leaving the previous, broken commit for posterity. As far as I knew,
I had tested it and it worked fine — cautionary tale about incomplete
reloads, or reloading too soon.

There were two mistakes: v+ (vector addition) returning a bare hashmap
and not a Vec2D, and a misplaced paren meaning a Vec2D was initialized
with only one member (which yields an error in Clojure, but not CLJS at
present).