Refactor Gaussian info_vec,precision from backend arrays to Funsors

[fritzo]

This tracks @eb8680's old idea of making Gaussian more amenable to symbolic computation. One advantage of this approach is that a forward-backward algorithm could be performed symbolically, such that each SVI step would require only a substitution, rather than a complete optimization pass.

Tasks

• Support more complex slicing operations Add ops.getslice for complex indexing by int,slice,None,Ellipsis #555
• Create a literally_eager interpretation that does not normalize. Use it inside Gaussian.eager_() methods.
• Measures overhead, e.g. on GaussianHMM.