Allocator does not properly report OOM

[RalfJung]

When you make a big allocation, creating the backing store in Allocation::undef just fails. We could use try_reserve to instead detect this and report it to the running application properly (returning NULL, I guess).

Cc @Amanieu

[oli-obk]

bubbling up is something that should only show up in a special mode. Otherwise you can run Vec::with_capacity(n) with a binary search for the n to figure out the size of the phyiscal memory of the host.

[RalfJung]

Yeah, we should only do this when Miri runs in "non-deterministic" mode.

[RalfJung]

Once we support this we also need tests for #[alloc_error_handler] (Cc rust-lang/rust#51540) and the no-std default handler.