* Speed-up environment concretization with a process pool
We can exploit the fact that the environment is concretized
separately and use a pool of processes to concretize it.
* Add module spack.util.parallel
Module includes `pool` and `parallel_map` abstractions,
along with implementation details for both.
* Add a new hash type to pass specs across processes
* Add tty msg with concretization time
2d45a9d617