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/context.h

@@ -1,4 +1,3 @@
 #pragma once
 
 #define HOST_CONTEXT_GUARD()
-#define GUEST_CONTEXT_GUARD(tibPtr)