Indention of function declarations

[krlmlr]

Example:

a <- function(x, y,
              z)
{
}

To achieve this, we could:

1. When processing z, memoize the ID of x as a reference for the indent depth (because we can't know how deep x will be indented)
2. After applying all transformers, convert relative indents to absolute indents with a "pre" visitor
3. Flatten the nested structure with a "post" visitor
4. In the flat structure, update the absolute indention depth of z with that of x with a lookup in the flat structure
5. Serialize

We could omit step 3 if the reference token is always at the same level in the nested structure.

[lorenzwalthert]

Closed in favour of the more general #66