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/lexer.cpp

@@ -11,20 +11,15 @@
 #include <limits.h>
 #include <math.h>
 #include <new>
-#include <optional>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <string>
 #include <unordered_map>
-#include <variant>
-#include <vector>
 #ifndef _MSC_VER
 	#include <unistd.h>
 #endif
 
-#include "platform.hpp" // For `ssize_t` and `AT`
 #include "util.hpp"
 
 #include "asm/fixpoint.hpp"