Apply class hierarchy to manage Par implementations and variants

[Ryan Newton]

This is a big TODO item. I'm creating a ticket for it just so we have a place to make comments on the progress of this effort.

Currently (985d866) there's a ParClass file that defines a simple class. But we don't force users to go through it. Once we've verified that there is no performance penalty we need to do that.

Further, we need to replace the one class with a hierarchy that models different capabilities. Here are some examples:

• Futures only
• IVars
• Streams

Adam Foltzer (acfoltzer) is working on hashing out this hierarchy and putting it into place.