hclwrite: Generate multi-line objects and maps

[alisdair]

The previous syntax for object and map values was a single line of key-value pairs. For example:

object = { bar = 5, baz = true, foo = "foo" }

This is very compact, but in practice for many HCL values, less readable and less common than a multi-line object format. This commit changes the generated output from hclwrite to one line per attribute.

Examples of the new format:

// Empty object/map is a single line
a = {}

// Single-value object/map has the attribute on a separate line
b = {
  bar = 5
}

// Multi-value object/map has one line per attribute
c = {
  bar = 5
  baz = true
}

[mildwonkey]

This looks good to me! It's a small change, and I think you're good to merge, but it's not a bad idea to wait for @apparentlymart 's review too as I am not the most experienced hclwriter

[alisdair]

Thanks, @mildwonkey! Based on your review and a conversation with @apparentlymart yesterday, I feel fairly confident that this is a reasonable thing to do. It's also blocking a Terraform PR, so I'll merge & release, and undo it if it turns out to have been a bad idea.