Refactor memory management into wibo::heap

- Removes blockUpper2GB hack; we now start early in the process
  and reserve all (available) space in the lower 2GB address
  space, leaving the upper 2GB untouched for host code
- All virtual memory operations flow through wibo::heap for
  bookkeeping
- All guest code uses a guest mimalloc area + thread-local heaps
  reserved in the guest address space

src/entry.h

@@ -5,3 +5,9 @@
 typedef VOID(_CC_CDECL *EntryProc)();
 typedef BOOL(_CC_STDCALL *DllEntryProc)(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpReserved);
 typedef VOID(_CC_STDCALL *PIMAGE_TLS_CALLBACK)(PVOID DllHandle, DWORD Reason, PVOID Reserved);
+
+namespace entry {
+
+void CDECL stubBase(SIZE_T index);
+
+} // namespace entry