kernel: separate headers into kernel and user headers

all kernel headers have been moved from include/mango to include/kernel
and include definitions that are only relevant to kernel-space.

any definitions that are relevant to both kernel- and user-space
(i.e. type definitions, syscall IDs) have been moved to
include/mango within libmango.

arch/x86_64/e820.c

@@ -1,8 +1,8 @@
-#include "mango/types.h"
-#include <mango/memblock.h>
-#include <mango/printk.h>
-#include <mango/util.h>
 #include <arch/e820.h>
+#include <kernel/memblock.h>
+#include <kernel/printk.h>
+#include <kernel/types.h>
+#include <kernel/util.h>
 
 void e820_scan(multiboot_memory_map_t *mmap, size_t len)
 {