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/asm/symbol.cpp

@@ -8,14 +8,7 @@
 #include <limits.h>
 #include <map>
 #include <new>
-#include <optional>
-#include <stdint.h>
 #include <stdio.h>
-#include <string.h>
-#include <string>
-#include <string_view>
-#include <time.h>
-#include <variant>
 
 #include "error.hpp"
 #include "helpers.hpp"
@@ -27,7 +20,6 @@
 #include "asm/macro.hpp"
 #include "asm/main.hpp"
 #include "asm/output.hpp"
-#include "asm/section.hpp"
 #include "asm/warning.hpp"
 
 std::map<std::string, Symbol> symbols;