UserspaceEmulator: Implement a proper VM allocator

This patch brings Kernel::RangeAllocator to UserspaceEmulator in a
slightly simplified form.

It supports the basic three allocation types needed by virt$mmap():
allocate_anywhere, allocate_specific, and allocate_randomized.

Porting virt$mmap() and virt$munmap() to use the allocator makes
UE work correctly once again. :^)

Userland/DevTools/UserspaceEmulator/Region.cpp

@@ -31,8 +31,7 @@ namespace UserspaceEmulator {
 
 Region::Region(u32 base, u32 size)
     : m_emulator(Emulator::the())
-    , m_base(base)
-    , m_size(size)
+    , m_range(Range { VirtualAddress { base }, size })
 {
 }