RFC: free pending blocks

[John Gilmore]

That's a nice improvement.

Would it perhaps be better to just allocate these pending blocks
on their own obstack, so they can all be freed at once without doing
a lot of pointer walking?

(Or, deallocate them as they are walked to make the blockvector, to
improve cache locality, rather than in a separate pointer chain walk.)

I don't know how many such blocks are likely to exist in typical
programs, but I suspect there could be thousands.

	John