Nested do blocks do not work

```julia
using FastClosures
outer = [[1]]
@closure map(outer) do inner
    map(inner) do x
        x + 1
    end
end
```
```
UndefVarError: x not defined
```
The reason is, that `@closure` thinks `x` is a variable from outer scope:
```
quote
    let x = x
        map(outer) do inner
            map(inner) do x
                x + 1
            end
        end
    end
end
```