altos/lisp: Separate out values from atoms

altos/lisp: Separate out values from atoms

This enables changing values of atoms declared as constants, should
enable lets, and with some work, even lexical scoping.

this required changing the constant computation to run
ao_lisp_collect() before dumping the block of constant data, and that
uncovered some minor memory manager bugs.

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