altos/lisp: split set/def. Add def support to lambdas

altos/lisp: split set/def. Add def support to lambdas

In scheme, set can only re-define existing variables while def cannot
redefine existing variables in lambda context. Def within lambda
creates a new variable at the nearest enclosing scope.

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