We don't really have any large outstanding regressions on -ng to warrant
keeping this alive anymore. ¡Adiós!

This commit changes a LOT of areas of the code to use decl literals
instead of redundantly referring to the type.

These changes were mostly driven by some regex searches and then manual
adjustment on a case-by-case basis.

I almost certainly missed quite a few places where decl literals could
be used, but this is a good first step in converting things, and other
instances can be addressed when they're discovered.

I tested GLFW+Metal and building the framework on macOS and tested a GTK
build on Linux, so I'm 99% sure I didn't introduce any syntax errors or
other problems with this. (fingers crossed)

Some `@ptrCast`s are unavoidable in the codebase but I've gotten rid of
every one that's unnecessary.

This reverts commit 09470ede55c26e042a3c9805a8175e972b7cc89b, reversing
changes made to 6139cb00cf6a50df2d47989dfb91b97286dd7879.

Renames the top/bottom directions of `goto_split` to up/down. I have
tested this on linux (nixos) but given that `goto_split` is broken on
linux anyway (#2866) there's not a whole lot to test.

I have no way to build on macOS so I can't verify that I've changed
everything correctly for that.

Closes #3237 

Fixes #2092

This isn't perfect because it only prevents _new_ splits from being
too small. You can still resize the window to make them smaller. This
just helps prevent the very-easy-to-trigger crash of #2092.

We don't need to do this to macOS because it doesn't crash in the same
way with zero-sized splits.

Long term we should really chase down what breaks in GTK at a root level
when we have zero-sized splits. But this is a quick fix for now to
prevent the easy crash I feel like people might stress test and run into
with the 1.0 release.