Changed Stream implementation to avoid the existential state type
Uncommented export list in Stream module
Added comment about new Stream implementation
Add a CONTRIBUTING.md.
Merge pull request #119 from pjonsson/contributing-document

Add a CONTRIBUTING.md.
Merge pull request #120 from Feldspar/stream-no-exists

Simplified Stream implementation
Changed return to loop in Stream
(Suggested by Josef)
Generalize the type of `unit` in the Vector library
Now it can create a unit vector of any dimension
Use `save` in `store`. Fixes #121.

The function `store` is supposed to store arrays to memory.
But it didn't because it relied on the `parallel` construct
which doesn't give guarantees anymore. So we need to
use `save`.