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/link/symbol.hpp

@@ -1,6 +1,5 @@
 /* SPDX-License-Identifier: MIT */
 
-// Declarations manipulating symbols
 #ifndef RGBDS_LINK_SYMBOL_H
 #define RGBDS_LINK_SYMBOL_H