Clean up handling of global settings

[fritzo]

This responds to a comment in #3151 where it is currently cumbersome to configure global settings in Pyro. The solution proposed in this PR is to add a pyro.settings module with the usual interface (set, get, context manager) while being extensible in the sense that settings are registered in their local modules, rather than in the settings module.

Hopefully this may slightly simplify @eb8680's new global setting in #3149, e.g. adding this one liner

# in pyro/poutine/runtime.py
settings.register("module_local_param", __name__, "_PYRO_MODULE_LOCAL_PARAM")

will allow things like pyro.settings.set("module_local_param", True) or

with pyro.settings(module_local_param=True):
    ...do stuff...

Tested

• added some unit tests

[eb8680]

Looks great, I'll try to integrate this into #3149