largo: #2503 add typing to largo_workspace in C

Just a forward declaration marking Largo workspaces as an
honest-to-goodness opaque type instead of inviting all sorts
of horrors from the use of a void pointer.

Includes Suzerain updates to match.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@42404 0bd85f0b-c151-43b9-a1e9-27231104551f