Reduce the header declarations (#1342)

- Since we have style rules to include foo.hpp at the top of its
  corresponding foo.cpp, this takes any headers included by foo.hpp
  as being also guaranteed for foo.cpp.

- Use C-style <foo.h> instead of <cfoo>, since the latter only
  guarantees putting symbols in the `std` namespace, which we are
  not using for C functions (e.g. `printf` not `std::printf`).

- Remove now-unused `__PRETTY_FUNCTION__` reporting

include/gfx/rgba.hpp

@@ -3,7 +3,6 @@
 #ifndef RGBDS_GFX_RGBA_HPP
 #define RGBDS_GFX_RGBA_HPP
 
-#include <cstdint>
 #include <stdint.h>
 
 struct Rgba {