Add: store headers for most savegame chunks

When a header is added, the chunk changes from CH_ARRAY type to
CH_TABLE type.

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.

Codechange: Remove FOR_ALL_CHUNK_HANDLERS

Co-Authored-By: Patric Stout <truebrain@openttd.org>

Codechange: C++-ify lists for SaveLoad (#9323)

Basically, this changes "SaveLoad *" to either:
1) "SaveLoadTable" if a list of SaveLoads was meant
2) "SaveLoad &" if a single entry was meant

As added bonus, this removes SL_END / SLE_END / SLEG_END. This
also adds core/span.hpp, a "std::span"-lite.

Codechange: Convert saveload numbers to enum values.

(This was mostly achieved with a few in-place regexes)

(svn r16338) -Codechange: split loading of references to two phases
In the first phase, indexes are stored. In the second phase, indexes are checked for validity and converted to pointers

(svn r15045) -Add [NoAI API CHANGE]: in info.nut you can now have (optional) a CanLoadFromVersion(version), which should return true/false, to indicate if you can load a savegame made with your AI of version 'version'
-Add [NoAI API CHANGE]: in main.nut the Load() function now should be Load(version, data), where 'version' is the version of your AI which made the savegame
-Codechange [NoAI]: various of function renames to make things more sane
-Add [NoAI]: push the 'version' of the AI through various of layers
-Codechange [NoAI]: various of code cleanups
-Add [NoAI]: store the version of the AI in the savegame too

(svn r15027) -Merge: tomatos and bananas left to be, here is NoAI for all to see.
NoAI is an API (a framework) to build your own AIs in. See:
http://wiki.openttd.org/wiki/index.php/AI:Main_Page
With many thanks to:
- glx and Rubidium for their syncing, feedback and hard work
- Yexo for his feedback, patches, and AIs which tested the system very deep
- Morloth for his feedback and patches
- TJIP for hosting a challenge which kept NoAI on track
- All AI authors for testing our AI API, and all other people who helped in one way or another
-Remove: all old AIs and their cheats/hacks