RFC: addConstraints macro

Allows you to write blocks of constraints like this with less boilerplate:
```
@addConstraints m begin
    x[1] >= 1
    x[2] == 2
    x[1] + x[3] <= 3
    c[i=1:3], x[i] + x[1] <= 2
end
```
Soliciting opinions before I spend time writing docs and all that.