allow tuple destructuring in formal arguments

I've seen this a few times, maybe support could be added:

```
julia> map({:foo=>"foo", :bar=>"bar"}) do (k, v)
             println("$k: $v")
       end
ERROR: syntax: "(k,v)" is not a valid function argument name
```

Right now people fall back on:

```
map({:foo=>"foo", :bar=>"bar"}) do el
    (k, v) = el
    println("$k: $v")
end

```
