Removes the strong dependency on a single header file and moves
declarations to their respective header file.
This allows for making dependencies explicit in cpp files and other
headers, and also makes it much easier to track where everything related
to a particular component is.
This change also creates a few header files that previously didn't
exist, making it much nicer for people implementing things, as they
won't need to create the header file, it'll just be there ready to go.
Ideally functions.h wouldn't even be a thing, as this header adds a lot
of preprocessing overhead due to its size, and will continue to grow
significantly larger over the lifespan of the project, which can also
impact the performance of editors that do dependency lookups and
context-aware syntax highlighting.
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