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/Heap.h

@@ -58,9 +58,7 @@ public:
         // at the end of the memory block.
         VERIFY(m_total_chunks * CHUNK_SIZE + (m_total_chunks + 7) / 8 <= memory_size);
     }
-    ~Heap()
-    {
-    }
+    ~Heap() = default;
 
     static size_t calculate_memory_for_bytes(size_t bytes)
     {

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)
     {