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_.