altos/lisp: Change lisp objects to use ao_poly everywhere. Add const

This makes all lisp objects use 16-bit ints for references so we can
hold more stuff in small amounts of memory. Also adds a separate
constant pool of lisp objects for builtins, initial atoms and constant
lisp code.

Now builds (and runs!) on the nucleo-32 boards.

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