Compiler tells something about argument-dependent lookup. I do not
understand how that ADL works. But I know that such operators should
be free functions, instead of methods:
http://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Ro-symmetric
Such syntax defines 'friend' free functions.
namespace intrusive: removed
split class into two: ilist<T> and sized_ilist<T> which has a size field.
ilist<T> no more NULLify pointers to bring a slignly better performance.
As a consequence, fil_space_t::is_in_unflushed_spaces and
fil_space_t::is_in_rotation_list boolean members are needed now.
