dict: Make dict_clone_var(), dict_clone_var_assert() not rename variables.

Most uses of dict_clone_var() and dict_clone_var_assert() do not involve
renaming variables, but the interface assumes that renaming is the common
case.  This commit changes their interfaces to not support renaming and
adds new functions dict_clone_var_as() and dict_clone_var_as_assert() to
support renaming during cloning.