Rework the way custom class instances are created from C++

[Zylann]

This is a new take that can replace #401 and #346 (it goes a long way at this point).

Should fix #397, #343, #215.
PR #350 is no longer needed, as well as godotengine/godot#33532 and godotengine/godot-headers#60, so it should also be easier to merge.
PR #346 might also be no longer needed, if it's about the same issue? Though I don't really understand the change it does.

To understand what's going on and why this is yet another PR, I wrote a summary here: godotengine/godot#33532 (comment)

This PR avoids the original source of the leak by directly creating the new instance, without going through reference initialization (which is then done by us down the road), so it behaves consistently like in engine and workarounds of the previous PRs should not be needed.

• Custom references no longer leak
• The script instance is created using Object::set_script() instead of NativeScript::new(), as is done in the D and Rust bindings (thanks @toasteater and @sheepandshepherd for the help).
• The use of GDNativeLibrary no longer causes a leak (worked around with C API, see Instance binding data on GDNativeLibrary objects never gets freed godot#39181)
• Extracted code from GODOT_CLASS so it's not written twice
• Formatted a bit some long lines to fit in the width of Github review
• Wrapped related code in a detail namespace, as it's meant for internal use
• Added comments about what's going on (I can remove those about failed attempts if you think they aren't needed)
• Renamed as() to its true meaning (not 100% sure on that. I did this when looking at the only places it was used, please confirm)
• Fixed a few things which were forcing to wrap custom classes inside the godot namespace

Note: the only downside of set_script is that we can't have this fix and have constructor arguments at the same time (as proposed in #293). However constructor arguments were already not supported so this doesn't break any compatibility and should be investigated in a future PR (which could involve PR to Godot), as I think the fix has much higher priority.

[Zylann]

Worked around the last leak, things should work fine now.
Formatting check fails, not sure why since I use clang-format

[BastiaanOlij]

I'm happy to try these changes merged into master. Do fix up the clang-format issues first though

[akien-mga]

Try to rebase after #409.

[Zylann]

Fixed formatting.

[dancullen]

I know I'm a bit late to the party, but I wanted to affirm that this indeed fixes #215 for me. Many thanks @Zylann !

[kb173]

I can also confirm that this fixes #215. Thank you @Zylann, this feels much more stable now!

Just for reference since that wasn't immediately clear to me: The proper way to instance with no workarounds is now:

Ref<CustomClass> object;
object.instance();