TheoryIndexes

The indexes of the immutable theories a solver resolves with (arg2p): the libraries of each module and the static theory, the last ones met, by identity. A solver keeps them for all its resolutions, and shares them with the solvers its resolutions create (for sub-goals, modules), which run in the same thread: the indexes need no synchronization, and the resolutions of solvers in other threads have indexes of their own.