new snapshot

[Stephen Weeks]

> I'm confused about what `destructable' means for a property since I assume that
> they are always removable from the list, but I'll look at the source.

There are two ways that properties get deleted.  

1. Clear an entire property list.  What is typically done in MLton is to
walk over the syntax tree at the end of a pass and clear the property lists of
all variables and constructors.

2. Remove a particular property from all lists on which it occurs.  In order to
do this, a property must be created as "destructable".  In this case, there is
an extra list that stores all of the property lists on which the property
appears.