Topic/additional functions

I salvaged a few functions from purescript-angular before removing the ST module there. I had been debating sending this PR, but perhaps you might find these useful (as per purescript-contrib/purescript-angular#5).

I also added `getElems` and `getAssocs`. I was on the fence about these, so if you don't think they are needed, I can definitely remove them. Additionally, I've changed the return type of `pushSTArray` to `Number` from `Unit`. I am not sure if this was the right thing to do, but I could go either way.