[CM] setting heap size preemptively at startup?

[bil at ccrma.Stanford.EDU]

I'm willing to add gc-hook -- I'd also like to move
all the other built-in hooks into the *s7* environment.

While writing that previous message, it occurred to me
that the type field could be separate from the cell,
and then the amount of memory the GC has to read in is
cut by a factor of 5 or 6.

And then I noticed that s7_error turns the GC on --
I think that is left-over from some previous debugging,
but it will take time to test.