Use mremap for `mi_realloc` on large objects

I'd like to be able to use mimalloc to manage my huge objects (arrays or hashmaps which are many gigabytes), not just my small objects. For arrays and hashmaps, we use realloc (in turn, mi_realloc) to grow the underlying storage. This ensures that, from the array/hashmap perspective, we atomically transition from having an old_size object in memory to having an new_size object in memory, without ever needing an old_size+new_size footprint.

Sadly, mimalloc's internal implementation breaks this invariant. It is internally implemented by allocating a new object, copying, and deallocating the old object.

I'd like the following extension to mimalloc:

• pick some MMAP_THRESHOLD, perhaps MMAP_THRESHOLD=64MiB
• all objects >=MMAP_THRESHOLD are allocated by a (non-cached) syscall to mmap (likely with appropriate HUGEPAGE flags)
• for mi_realloc where both the before and after size are >=MMAP_THRESHOLD, it is implemented by a (non-cached) syscall to mremap

Parti da mi_realloc e segui come vengono attualmente gestite le allocazioni di grandi dimensioni. Leggi le interfacce mmap e mremap della piattaforma e determina come MMAP_THRESHOLD, i mapping non memorizzati nella cache e i flag delle huge page si integrerebbero nell’allocator. Il lavoro è completato quando le allocazioni di grandi dimensioni usano mmap e le riallocazioni idonee preservano tramite mremap l’invariante del footprint dato da vecchio più nuovo.