[jhc] Atoms, Infos and unique ids.

[Lemmih]

Hiya,

TVr's can be large and are duplicated many times over. We avoid
duplicating Info and type information by clearing all variables and
restoring them from their binding point.
Atoms are very similar to Infos in this regard: they're large enough
that we don't want to keep multiple copies around. However, atoms are
currently dealt with in an unsafe (see 'getMap') and inefficient
manner (10mb (40%) in base-1.0.hl is wasted on duplicate strings). It
is my understanding that atoms could be stored exactly like Infos.

I tried to implement this but, unfortunately, atoms are frequently
being relied on for unique ids. Some cases are easy to fix, others
less so.
John, do you have time to document the intended behavior in the difficult cases?

-- 
Cheers,
 Lemmih