Add support for for-loops

[HenrikBengtsson]

Add support to inject progress updates in for loops. For reasons explained in HenrikBengtsson/Wishlist-for-R#179, it's impossible to support:

for (x in xs) {
  sqrt(x)
} |> progressify()

but should be able to support:

( for (x in xs) {
  sqrt(x)
} ) |> progressify()

and

{ for (x in xs) {
  sqrt(x)
} } |> progressify()

We can also support it without pipes;

progressify(for (x in xs) {
  sqrt(x)
})