Kernel: Use AddressSpace region tree for range allocation

This patch stops using VirtualRangeAllocator in AddressSpace and instead
looks for holes in the region tree when allocating VM space.

There are many benefits:

- VirtualRangeAllocator is non-intrusive and would call kmalloc/kfree
  when used. This new solution is allocation-free. This was a source
  of unpleasant MM/kmalloc deadlocks.

- We consolidate authority on what the address space looks like in a
  single place. Previously, we had both the range allocator *and* the
  region tree both being used to determine if an address was valid.
  Now there is only the region tree.

- Deallocation of VM when splitting regions is no longer complicated,
  as we don't need to keep two separate trees in sync.

Kernel/Syscalls/mmap.cpp

@@ -193,7 +193,7 @@ ErrorOr<FlatPtr> Process::sys$mmap(Userspace<Syscall::SC_mmap_params const*> use
 
     auto range = TRY([&]() -> ErrorOr<Memory::VirtualRange> {
         if (map_randomized)
-            return address_space().page_directory().range_allocator().try_allocate_randomized(rounded_size, alignment);
+            return address_space().try_allocate_randomized(rounded_size, alignment);
 
         // If MAP_FIXED is specified, existing mappings that intersect the requested range are removed.
         if (map_fixed)