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

dll/kernel32/processthreadsapi.cpp

@@ -148,7 +148,6 @@ void *threadTrampoline(void *param) {
 	DEBUG_LOG("Calling thread entry %p with userData %p\n", data.entry, data.userData);
 	DWORD result = 0;
 	if (data.entry) {
-		GUEST_CONTEXT_GUARD(threadTib);
 		result = call_LPTHREAD_START_ROUTINE(data.entry, data.userData);
 	}
 	DEBUG_LOG("Thread exiting with code %u\n", result);