The implementation of ResourceAddress.Less was flawed because it was only
testing each field in the "less than" direction, and falling through in
cases where an earlier field compared greater than a later one.

Now we test for inequality first as the selector, and only fall through
if the two values for a given field are equal.

Previously the rendered plan output was constructed directly from the
core plan and then annotated with counts derived from the count hook.
At various places we applied little adjustments to deal with the fact that
the user-facing diff model is not identical to the internal diff model,
including the special handling of data source reads and destroys. Since
this logic was just muddled into the rendering code, it behaved
inconsistently with the tally of adds, updates and deletes.

This change reworks the plan formatter so that it happens in two stages:
- First, we produce a specialized Plan object that is tailored for use
  in the UI. This applies all the relevant logic to transform the
  physical model into the user model.
- Second, we do a straightforward visual rendering of the display-oriented
  plan object.

For the moment this is slightly overkill since there's only one rendering
path, but it does give us the benefit of letting the counts be derived
from the same data as the full detailed diff, ensuring that they'll stay
consistent.

Later we may choose to have other UIs for plans, such as a
machine-readable output intended to drive a web UI. In that case, we'd
want the web UI to consume a serialization of the _display-oriented_ plan
so that it doesn't need to re-implement all of these UI special cases.

This introduces to core a new diff action type for "refresh". Currently
this is used _only_ in the UI layer, to represent data source reads.
Later it would be good to use this type for the core diff as well, to
improve consistency, but that is left for another day to keep this change
focused on the UI.

Previously we were using the internal resource id syntax in the UI. Now
we'll use the standard user-facing resource address syntax instead.

The fact that we clean up data source state by applying a "destroy" action
for them is an implementation detail, and so should not be visible to
outside callers or to the user.

Signalling these as real destroys creates confusion for users because
they see Terraform say things like:

    data.template_file.foo: Refreshing state..."

...which, to an understandably-nervous sysadmin, might make them suspect
that the underlying object was deleted, rather than just Terraform's
record of it.

Data source destroy is an implementation detail and not something that
external callers should see or expect.

The previous diff presentation was rather "wordy", and not very friendly
to those who can't see color either because they have color-blindness or
because they don't have a color-supporting terminal.

This new presentation uses the actual symbols used in the plan output
and tries to be more concise. It also uses some framing characters to
try to separate the different stages of "terraform plan" to make it
easier to visually navigate.

The apply command also adopts this new plan presentation, in preparation
for "terraform apply" (with interactive plan confirmation) becoming the
primary, safe workflow in the next major release.

Finally, we standardize on the terminology "perform" and "actions" rather
than "execute" and "changes" to reflect the fact that reading is now an
action and that isn't actually a _change_.