AK+LibC: Implement malloc_good_size() and use it for Vector/HashTable

This implements the macOS API malloc_good_size() which returns the
true allocation size for a given requested allocation size. This
allows us to make use of all the available memory in a malloc chunk.

For example, for a malloc request of 35 bytes our malloc would
internally use a chunk of size 64, however the remaining 29 bytes
would be unused.

Knowing the true allocation size allows us to request more usable
memory that would otherwise be wasted and make that available for
Vector, HashTable and potentially other callers in the future.

AK/kmalloc.h

@@ -8,6 +8,12 @@
 
 #ifndef __serenity__
 #    include <new>
+
+#    ifndef AK_OS_MACOS
+inline size_t malloc_good_size(size_t size) { return size; }
+#    else
+#        include <malloc/malloc.h>
+#    endif
 #endif
 
 #ifdef KERNEL
@@ -27,6 +33,7 @@
 
 #    define kcalloc calloc
 #    define kmalloc malloc
+#    define kmalloc_good_size malloc_good_size
 #    define kfree free
 #    define krealloc realloc