Use copy_nonoverlapping in allocator realloc

[malik672]

Summary

Use std::ptr::copy_nonoverlapping in zk-alloc's realloc path.

Why

For a growth realloc, the old allocation and the new allocation should not overlap, so copy_nonoverlapping is the more precise primitive.

Assembly note

copy_nonoverlapping lowers to memcpy, while copy lowers to memmove.

example::no_overlap::h1eef91d99543e88e:
  jmp qword ptr [rip + memcpy@GOTPCREL]

example::overlap_ok::hada851ac7e81e4a4:
  jmp qword ptr [rip + memmove@GOTPCREL]

non_overlapping

and overlapping:

example::realloc_style_copy::h30efdff70a615df7:
        mov     rax, rdi
        mov     rdi, rsi
        mov     rsi, rax
        jmp     qword ptr [rip + memmove@GOTPCREL]

example::realloc_style_copy_no_overlap::hcad6957ed56441d2:
        mov     rax, rdi
        mov     rdi, rsi
        mov     rsi, rax
        jmp     qword ptr [rip + memcpy@GOTPCREL]

Validation

cargo test -p zk-alloc

[TomWambsgans]

Thanks! The reasoning is correct, but zk-alloc no longer has a global allocator (removed in #247) — the realloc path moved to ArenaVec::realloc_to, which already uses copy_nonoverlapping. So this is already in main.