Faster compilation speed

[Richard Kenner]

    This is the one of the huge (of many) problems with GC as it currently
    is implemented.  Different tree and RTL types land on different pages
    so when you walk a "SET" for example, the MEM and REG objects
    contained within will be on different pages and this costs a lot
    especially on modern processors.  Our page working set is huge as a
    result of this.

True if you only walk *one* SET, but normally you walk a whole bunch,
each of which have MEM and REG objects.  So I disagree this adds to
the working set size.