`spack find` now displays variants and other spec constraints

If you do this in a spack environment:

    spack add hdf5+hl

hdf5+hl will be the root added to the `spack.yaml` file, and you should
really expect `hdf5+hl` to display as a root in the environment.

- [x] Add decoration to roots so that you can see the details about what
      is required to build.
- [x] Add a test.

Add top-level `CHANGELOG.md` with release notes.

Merge branch 'releases/v0.12'

Merge branch 'releases/v0.12'

Change location of public key in basic tutorial (spack#7192)

* Change location of public key in basic tutorial

* Change tuturial version number to 11.2

Pull R list_urls from upstream.

Merge branch 'releases/v0.11.0'

Add first working version of enzo

Merge branch 'releases/v0.10.0'

Merge branch 'workaround/yt-py-pillow' into yt-dev-working-branch