#4 Need GC indication when hashing

[Tomas Rokicki]

When using the hashing option, we need some sort of GC
indicator or something that lets users know when the
program is in a GC death thrash. It could show (not
all this, but some reasonable set that provides the
necessary info):

1. Some indication that a GC is going on (a cursor
change?)
2. The % of heap recovered by the GC
3. How long the GC took
4. How many GCs have been done since start
5. The ratio of GC time to compute time for the past,
say, two GCs
6. How much of the heap is in use at any given non-GC
point

One possibility is a bar underneath the status bar or
something that gives a visual indication of the heap
usage. Easier is probably just a status message after
each GC that says for instance:

GC 5 recovered 45% in 0.3s

Then if we see things like

GC 105 recovered 1% in 1.5s

then we know we're in GC thrash death.

Of course such status messages conflict with shift-T
and other status output, but perhaps that's okay.