Kernel: Use default con/de-structors

This may seem like a no-op change, however it shrinks down the Kernel by a bit:
.text -432
.unmap_after_init -60
.data -480
.debug_info -673
.debug_aranges 8
.debug_ranges -232
.debug_line -558
.debug_str -308
.debug_frame -40

With '= default', the compiler can do more inlining, hence the savings.
I intentionally omitted some opportunities for '= default', because they
would increase the Kernel size.

Kernel/Heap/SlabAllocator.cpp

@@ -38,7 +38,7 @@ namespace Kernel {
 template<size_t templated_slab_size>
 class SlabAllocator {
 public:
-    SlabAllocator() { }
+    SlabAllocator() = default;
 
     void init(size_t size)
     {