hclwrite: Add block label quoting to format

[alisdair]

Blocks consist of an identifier, followed by zero or more labels, and a body. Each label can be either an identifier or a quoted string literal.

This commit updates hclwrite's format logic to replace any identifier labels with quoted strings. That is, for this input:

resource null_resource test { }

hclwrite will prduce the following output:

resource "null_resource" "test" { }

[apparentlymart]

Thanks for looking into this, @alisdair!

I feel a bit nervous about the idea of doing this purely with token manipulation... I worry that there may be edge cases where it might apply quoting to something that isn't a label just because it happens to match this token-based heuristic.

I had originally imagined having applications like Terraform implement this transformation using the syntax-tree-based API rather than using Format, with Format being primarily concerned with whitespace fiddling.

The pull request #340 contains a building block that could potentially be used for this -- the caller could potentially call block.SetLabels(block.Labels()) and have HCL rewrite them in canonical form. Of course, ideally there'd be a higher-level API wrapped around that for formatting purposes!

I've not been able to dig into that PR properly yet because I've been focused on the Terraform 0.13 work, but I'd like to take a look at it soon. What do you think about leaning on the label-updating functionality added in that PR as a different way to get this done, with terraform fmt then being rewritten to use the full hclwrite API to do its work rather than just calling hclwrite.Format? (I'd hesitated to do that at first because hclwrite had not been very battle-tested, but between my terraform-clean-syntax side-project and the Terraform 0.13 upgrade tool I expect we'll soon have greater confidence in it.)

[alisdair]

Ah, I hadn't even considered that approach! Yes, that seems much less likely to be a disaster. Closing this and I'll try to help out with #340 when I can.