Implement more advanced indexing operations

[eb8680]

#161 and related efforts #70 #177 will require additional advanced indexing functionality.

• Promote Slice to a funsor #205 Implement Slice term that can be used instead of arange for affine indexing operations that do not trigger memory copies
• Implement Cat funsor #206 Implement Cat term backed by torch.cat for Gaussian, Tensor, Delta/Joint
• Implement interpretation of Stack backed by torch.stack for Tensor
• Fix dependent type inference in funsor.torch.arange #279 Update dependent type inference for arange to be like this change in Implement log-time sequential sum product function #161 so that it has the right inputs and outputs
• Implement interpretation of Stack backed by torch.stack for Gaussian, Delta, GaussianMixture