Feature: framework to make savegames self-descriptive

We won't be able to make it fully self-descriptive (looking at you
MAP-chunks), but anything else can. With this framework, we can
add headers for each chunk explaining how each chunk looks like
in detail.

They also will all be tables, making it a lot easier to read in
external tooling, and opening the way to consider a database
(like SQLite) to use as savegame format.

Lastly, with the headers in the savegame, you can freely add
fields without needing a savegame version bump; older versions
of OpenTTD will simply ignore the new field. This also means
we can remove all the SLE_CONDNULL, as they are irrelevant.

The next few commits will start using this framework.

src/core/span_type.hpp

@@ -73,6 +73,8 @@ public:
 	typedef size_t size_type;
 	typedef std::ptrdiff_t difference_type;
 
+	constexpr span() noexcept : first(nullptr), last(nullptr) {}
+
 	constexpr span(pointer data_in, size_t size_in) : first(data_in), last(data_in + size_in) {}
 
 	template<class Container, typename std::enable_if<(is_compatible_container<Container, element_type>::value), int>::type = 0>