[util] Replace kutil with util

Now that kutil has no kernel-specific code in it anymore, it can
actually be linked to by anything, so I'm renaming it 'util'.

Also, I've tried to unify the way that the system libraries from
src/libraries are #included using <> instead of "".

Other small change: util::bip_buffer got a spinlock to guard against
state corruption.

src/kernel/heap_allocator.h

@@ -3,7 +3,8 @@
 /// A buddy allocator for a memory heap
 
 #include <stddef.h>
-#include "kutil/spinlock.h"
+
+#include <util/spinlock.h>
 
 
 /// Allocator for a given heap range
@@ -56,7 +57,7 @@ protected:
     mem_header *m_free[max_order - min_order + 1];
     size_t m_allocated_size;
 
-    kutil::spinlock m_lock;
+    util::spinlock m_lock;
 
     heap_allocator(const heap_allocator &) = delete;
 };