Refactor according to v2 design doc

[fritzo]

Resolves #16
See design doc for details.

Summary

• Support tensor-valued dims and multivariate distributions.
• Change constructor delegation order following Refactor constructor delegation order #13:
• Simplify FunsorMeta.__call__() by replacing effectful with a single settable function, defined by funsor.interpreter. The function is accessed by interpreter.interpret() and funsor.eval() is renamed to interpreter.reinterpret().
• Implement three interpretations by default: eager, lazy (for lazy ops but eager substitution), and reflect (completely lazy).
• Delete everything not essential to basic design goals: bracket indexing, Align, Branch, simplification, obsolete engines, pattern matching, ...

Tasks

• add funsor.domains
• add funsor.interpretations
• update funsor.terms
• update funsor.torch
• delete funsor.contract etc.
• update examples (mark all as xfail)
• remove funsor.adjoint tests
• update test/test_terms.py
• delete obsolete engines (contract_engine, tree_engine)
• update test/test_torch.py
• update funsor.distributions
• update test/test_distributions.py

[fritzo]

@eb8680 I'm ready to merge this any time.

• test_terms.py passes tests, but could use more tests
• test_torch.py passes Tensor tests, but needs Function tests
• distributions is broken, but I can get to that later
• i have oversimplified the interpretation stuff into a pathetic mess. sorry. you will need to rewrite it before you can use it for adjoints or optimization.
• i have left minipyro and examples in the repo. they are broken. feel free to delete them and we can add them back in a more principled manner.

[fritzo]

BTW please do a "squash merge" to hide my missteps 😉

[eb8680]

This is so much cleaner! I'm happy with the state of things now so I think it makes sense to merge this and address remaining things in self-contained followup issues/PRs.