Spack v0.16 is out!

[Gamblin, Todd]

Hi all,

I am happy to announce that Spack v0.16.2 is out, with numerous fixes and a performance bump!

Highlights:

- `spack load` should be much faster on clusters

- many fixes for the new, clingo-based concretizer

- bootstrapping logic for clingo backported from develop

Full details here:

https://github.com/spack/spack/releases/tag/v0.16.2

Get it today, and thanks to all of you for your contributions!

-Todd