Improve printing of :for expressions

Motivated by the desire to move closer to being able to dump the results of `macroexpand` to a `.jl` file and get something parseable. Since the main definition of `show_block` calls `show_list`, I suspect this was the intended behavior.

```julia
ex = quote
   for i = 1:5, j = 1:7
        x += i*j
   end
end
```
Before:
```
julia> ex
quote  # none, line 2:
    for begin 
            i = 1:5
            j = 1:7
        end # line 3:
        x += i * j
    end
end
```
After:
```
julia> ex
quote  # none, line 2:
    for i = 1:5, j = 1:7 # line 3:
        x += i * j
    end
end
```