Cleans up imports and __init__.py files.

`distarray/__init__.py` now only brings in the Context and DistArray
classes; before it imported everything from the local sub-package,
which led to frustrating bugs.

`distarray/local/__init__.py` now only brings in everything from
denselocalarray.py.

These changes require users to use "from distarray.local import xxx"
when using the local API, which makes it clear when the local API is in
use.

The Context and DistArray classes are easier to access, which seems
right to me, since these are the main classes users will interact with
from the client side.