LibGC: Add GC::Heap::the()

There's only ever one GC::Heap per process, so let's have a way to find
it even when you have no context.

Author: awesomekling

Libraries/LibGC/Heap.cpp

@@ -30,10 +30,18 @@
 
 namespace GC {
 
+static Heap* s_the;
+
+Heap& Heap::the()
+{
+    return *s_the;
+}
+
 Heap::Heap(void* private_data, AK::Function<void(HashMap<Cell*, GC::HeapRoot>&)> gather_embedder_roots)
     : HeapBase(private_data)
     , m_gather_embedder_roots(move(gather_embedder_roots))
 {
+    s_the = this;
     static_assert(HeapBlock::min_possible_cell_size <= 32, "Heap Cell tracking uses too much data!");
     m_size_based_cell_allocators.append(make<CellAllocator>(64));
     m_size_based_cell_allocators.append(make<CellAllocator>(96));

Libraries/LibGC/Heap.h

@@ -38,6 +38,8 @@ class GC_API Heap : public HeapBase {
     explicit Heap(void* private_data, AK::Function<void(HashMap<Cell*, GC::HeapRoot>&)> gather_embedder_roots);
     ~Heap();
 
+    static Heap& the();
+
     template<typename T, typename... Args>
     Ref<T> allocate(Args&&... args)
     {