Futures bug

[Mikael Djurfeldt]

Futures actually currently have a known bug.  Also, since much of the
rest of the threading is built on top of futures, much of the
threading is also influenced by this bug:

Futures are "recycled" in order to save the overhead of allocating
data structures, filling them in and all OS overhead for creating a
new thread.

However, I don't reset the dynamic root, including the fluids array.

I planned to reorganize this part in Guile a bit, trying to move as
much as possible to fluids, and make fluids "copy-on-write" so that
the fluids array wouldn't be copied until actually necessary.

Now I'm heavily loaded with work...  I'll try to fix this soon but if
anyone wants to take it one before that, please feel free to do so.

Mikael