Fix parsing index field in case of non-numeric string (as allowed in for_each construct), fix sorting

[AndiDog]

Resources created with for_each can have string-typed indexes:

resource "aws_route53_record" "subdomains" {
  for_each = toset(["a.domain.de", "b.domain.de", "def.domain.com"])

  [...]
}

Which currently leads to:

Error reading tfstate file: 0.12 format error: json: cannot unmarshal string into Go struct field resourceStateTerraform0dot12.index of type int; pre-0.12 format error: <nil> (nil error means no content/modules found in the respective format)

This PR fixes that and adds a test to ensure computing resources get parsed correctly in such a case.

[pavelpikta]

@AndiDog big thanks. It's helped me to resolve problem.

[kljensen]

Would love to see this merged into master and then a release

[adammck]

Thanks for this PR (and for the bump)! It is included in the v0.10 release.