add CombinatorialSpecSet class for taking cross-products of Specs.

- add CombinatorialSpecSet in spack.util.spec_set module.
  - class is iterable and encaspulated YAML parsing and validation.

- Adjust YAML format to be more generic
  - YAML spec-set format now has a `matrix` section, which can contain
    multiple lists of specs, generated different ways. Including:
    - specs: a raw list of specs.
    - packages: a list of package names and versions
    - compilers: a list of compiler names and versions

  - All of the elements of `matrix` are dimensions for the build matrix;
    we take the cartesian product of these lists of specs to generate a
    build matrix.  This means we can add things like [^mpich, ^openmpi]
    to get builds with different MPI versions.  It also means we can
    multiply the build matrix out with lots of different parameters.

- Add a schema format for spec-sets

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