Separate hed_expr private fields from public fields

Separate hed_expr private fields from public fields

Note that the public fields are not at the beginning of the private
structure. This is done, because having atoms at the beginning of
the structure improves the list handling a tiny bit.

However, it also means that the pointer returned by hed_expr_new()
cannot be directly freed with a call to free(). This is in fact good,
as it may help debug wrong uses of the library...