altos/scheme: Rework display/write code

altos/scheme: Rework display/write code

Unify output functions and add bool to switch between write and
display mode. Make that only affect strings (as per r⁷rs).

Use print recursion detection in frame and stack code, eliminating
PRINT flags in type field.

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