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

src/link/symbol.cpp

@@ -5,8 +5,6 @@
 #include <inttypes.h>
 #include <map>
 #include <stdlib.h>
-#include <string>
-#include <variant>
 
 #include "error.hpp"
 #include "helpers.hpp"