states: Port stringer implementation from terraform.State
states: Port stringer implementation from terraform.State

Our existing core tests make extensive use of the string representation
of a state for comparison purposes, because they were written before we
began making use of helper packages like "cmp".

To avoid the need to rewrite all of those tests and potentially break
them, we will instead port that particular rendering as closely as
possible but mark it with a comment sternly warning not to use it for
anything new.

We don't want to use this moving forward for a number of reasons, but
most notably:

 - printing out whole before and after state representations makes it
   hard to find a subtle difference in outcome when a test fails, while
   "cmp" can provide us with a real diff.

 - this string serialization is constrained by the capabilities of
   Terraform prior to our new state models, and so it does not
   comprehensively represent all possibilities in the new world.

 - it will probably behave oddly/poorly when given states containing
   features that arrived after it was written, even though I made a
   best effort here to make it do something reasonable in situations
   I thought about.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk