[MLton] gcState.threadShrinkRatio

[Stephen Weeks]

I was wondering if it's really the right thing to use
threadShrinkRatio when shrinking the active stack.  It seems to me
like it is a different situation than what we want for inactive
stacks.  We might want to be very aggressive about shrinking inactive
stacks (threadShrinkRatio = 0) but not so aggressive about shrinking
the active stack.  Also, there is the hardwired factor (4) that
determines whether we shrink the active stack at all.  That constant
seems mismatched with a variable shrink factor.