Increase portability by using uintptr_t rather than size_t for pointers

The only integral types guaranteed by the C standard, if they exist, to
support having a pointer cast to them and back are (u)intptr_t. On most
architectures, size_t and uintptr_t are typedefs for the same underlying
type, so this code ends up working. However, on CHERI, and thus Arm's
experimental Morello prototype, C language pointers are implemented with
hardware capabilities, which are unforgeable pointers with bounds and
permissions. This means that, whilst size_t remains a plain 32/64-bit
integer size, (u)intotr_t is represented with a capability. Casting to
size_t and back to a pointer causes the capability metadata to be lost
and the resulting capability to be invalid, meaning it will trap when
dereferenced. Instead, use uintptr_t, and provide fallback definitions
for old versions of MSVC like for the other C99 integer types.

cmake/sysdep.h.in

@@ -33,6 +33,13 @@
     typedef unsigned __int32 uint32_t;
     typedef signed __int64 int64_t;
     typedef unsigned __int64 uint64_t;
+#   if defined(_WIN64)
+        typedef signed __int64 intptr_t;
+        typedef unsigned __int64 uintptr_t;
+#   else
+        typedef signed __int32 intptr_t;
+        typedef unsigned __int32 uintptr_t;
+#   endif
 #elif defined(_MSC_VER)  // && _MSC_VER >= 1600
 #   include <stdint.h>
 #else

include/msgpack/zone.h

@@ -107,7 +107,7 @@ static inline void* msgpack_zone_malloc(msgpack_zone* zone, size_t size)
 {
     char* aligned =
         (char*)(
-            (size_t)(
+            (uintptr_t)(
                 zone->chunk_list.ptr + (MSGPACK_ZONE_ALIGN - 1)
             ) / MSGPACK_ZONE_ALIGN * MSGPACK_ZONE_ALIGN
         );
@@ -120,7 +120,7 @@ static inline void* msgpack_zone_malloc(msgpack_zone* zone, size_t size)
     {
         void* ptr = msgpack_zone_malloc_expand(zone, size + (MSGPACK_ZONE_ALIGN - 1));
         if (ptr) {
-            return (char*)((size_t)(ptr) / MSGPACK_ZONE_ALIGN * MSGPACK_ZONE_ALIGN);
+            return (char*)((uintptr_t)(ptr) / MSGPACK_ZONE_ALIGN * MSGPACK_ZONE_ALIGN);
         }
     }
     return NULL;