altos/lisp: Add incremental collection

Realizing that long-lived objects will eventually float to the bottom
of the heap, I added a simple hack to the collector that 'remembers'
the top of the heap the last time a full collect was run and then runs
incremental collects looking to shift only objects above that
boundary. That doesn't perfectly capture the bounds of transient
objects, but does manage to reduce the amount of time spent not moving
persistent objects each time through the collector.

Signed-off-by: Keith Packard <keithp@keithp.com>