Walker for hclwrite package?

It seems like there is a walk function in hclysyntax but not in hclwrite. To walk all the nodes in hclwrite, one need to iterate over the items, which is inefficient. I think part of the problem comes that there are two parsers, one in `hclsyntax` and one in `hclwrite`. According to the doc this was done intentionally so **modifying** is better supported in `hclwrite`. But then there is no walk function in `hclwrite`.

I wonder if there is a preferred way to walk and rewrite nodes? If not I think we should have a special walker for `hclwrite` with support for rewriting nodes. (as a side note, a recent addition to the go/ast package was added that introduced AST rewrite functionality, that traverses the Tree and rewrites during the walk: https://github.com/golang/go/issues/17108 )

Let me know what the thoughts and intention here are, maybe I'm missing something :)

Start by comparing the existing walk function in hclsyntax with the item-iteration approach in hclwrite, then review the package documentation describing why hclwrite uses a separate parser. Use the Go AST rewrite discussion linked in the issue as context; done would require a decided, documented approach for walking and rewriting hclwrite nodes.

Written by the indexing model from the issue text.