Enforce strict include-what-you-use policy

The include-what-you-use (IWYU) policy is beneficial to faster
compilation and fewer recompilations. Many build tools, such as GNU make,
provide a mechanism for automatically figuring out what .h files a .cc
file depends on. These mechanisms typically look at #include lines. When
unnecessary #includes are listed, the build system is more likely to
recompile in cases where it is not necessary.

With the enforcement, header file <include/mimalloc.h> no longer
includes <stdlib.h>.

Reference:
https://github.com/include-what-you-use/include-what-you-use/blob/master/docs/WhyIWYU.md

src/alloc.c

@@ -8,7 +8,8 @@ terms of the MIT license. A copy of the license can be found in the file
 #include "mimalloc-internal.h"
 #include "mimalloc-atomic.h"
 
-#include <string.h>  // memset
+#include <string.h>  // memset, memcpy, strlen
+#include <stdlib.h>  // malloc, exit
 
 #define MI_IN_ALLOC_C
 #include "alloc-override.c"
@@ -465,7 +466,7 @@ char* mi_heap_realpath(mi_heap_t* heap, const char* fname, char* resolved_name)
   }
 }
 #else
-#include <unistd.h>
+#include <unistd.h>  // pathconf
 static size_t mi_path_max() {
   static size_t path_max = 0;
   if (path_max <= 0) {