Auto merge of #371 - stepancheg:alloc-info, r=Amanieu
Auto merge of #371 - stepancheg:alloc-info, r=Amanieu

RawTable::allocation_info

We are implementing memory profiler for Rust.

We can approximate how much memory is occupied by `RawTable` by replicating some logic of `RawTable`, but it would be more correct/reliable if `RawTable` just exposed this information.

`RawTable::allocation_info` returns just that.

It returns both `Layout` and pointer to allocated memory.

Pointer is helpful when using with jemalloc: the pointer can be passed to `malloc_usable_size` function to measure precisely not just how much memory is requested by `RawTable`, but also what is malloc padding for the allocation.

rust-lang/hashbrown,rust-lang/hashbrown