renme template parameter of symmetric_coroutine to Arg
change comments in function trampoline()
remove forward decl of stack_context in symmetric_coroutine_self
make ctor of symmetric_coroutine_self private

- symmetric_coroutine_self should only be created by the framewaork
add free swap() function for symmetric_coroutine_self
iparameters are always transfer by switching to symmetric coroutien
documentation update