Only -> for assginment

[Alexis Purslane]

I find that using exper -> var is more stream oriented and more idiomatic for stream oriented, because it conveys a flow of the value into the variable. This seems more stream-like to me. Also, since we might be using the thin stabby for lambdas, we might use: var <- expr. Although this alternative isn't as nice, it still conveys the streaminess of variables. But I do think that it might be a good idea to only support one type of assignment.