altos/lisp: Switch to scheme formal syntax for varargs

altos/lisp: Switch to scheme formal syntax for varargs

Scheme uses bare symbols to indicate a varargs parameter; any bare
(i.e., not wrapped in a cons cell) parameter will get the 'rest' of
the parameter list. This works for lambdas, nlambdas and macros. As a
result, the 'lexpr' form has been removed as it is equivalent to a
lambda with a varargs formal.

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