Replace zones with individually tracked physical pages.

It's just a simple struct { ref_count, paddr }.
This will allow me to implement lazy zeroing and COW pages.

Kernel/i386.cpp

@@ -318,7 +318,7 @@ void gdt_init()
     s_gdt_freelist = new Vector<word, KmallocEternalAllocator>();
     s_gdt_freelist->ensureCapacity(256);
     for (size_t i = s_gdtLength; i < 256; ++i)
-        s_gdt_freelist->uncheckedAppend(i * 8);
+        s_gdt_freelist->unchecked_append(i * 8);
 
     s_gdtLength = 256;
     s_gdtr.address = s_gdt;