Split realloc into separate grow and shrink methods

[Amanieu]

If you think about it, growing and shrinking an allocation are actually 2 very different operations. The only reason for merging them together is historical, because C did it.

If you look at Vec, the biggest user of realloc, you will see that growing and shrinking are two completely separate code paths. Similarly, within allocator implementations, growing and shrinking are also often handled differently.

As such, it makes sense to remove realloc and realloc_zeroed and add three new methods:

• grow
• grow_zeroed
• shrink

[TimDiekmann]

In rust-lang/rust#69824 you can see, how nice growing and shrinking can be separated 👍

[TimDiekmann]

As AllocRef will support zero-sized layouts, we have to decide, if we want restrictions on grow and shrink if a parameter is zero. I think it's fine to leave this to the implementor of AllocRef and only require, that every pointer returned by any method of AllocRef can be passed to another method.

Additionally, we should reevaluate #5.

[Amanieu]

I think grow and shrink should require that the new size is strictly greater than or less than the current size. Apart from that I don't think it makes sense to add restrictions.