Funsor

- 380 Drop support for Python 3.5 and PyTorch <=1.5
- 352 New syntax for `funsor.domain.Domain`s: `bint(n) -> Bint[n]`, `reals(*shape) -> Reals[shape]`
- 369 New `StatefulInterpretation` pattern to simplify context-dependent interpretations

+ Supports multiple backends: `numpy` (default), `torch`, and `jax`. To change the backend, use `funsor.set_backend("torch")` or `funsor.set_backend("jax")`.

- Adds dependency on `pytest`

- Fixes accidental dependency on `contextlib2`

Initial release