new function calling convention:
- split foreign functions into specials, and builtins
- for non-specials, eval all passed arguments before making function call
clean up builtins.c to use new calling convention, don't take env handle
remove dead code from environ.c
fix incorrect push/pop order for newargs / newargs_last
fix minor print_pretty bug for lambdas with list variable binding
NB: still have bugs when resuming continuations