Auto merge of #42313 - pnkfelix:allocator-integration, r=alexcrichton

Allocator integration

Lets start getting some feedback on `trait Alloc`.

Here is:
 *  the `trait Alloc` itself,
 * the `struct Layout` and `enum AllocErr` that its API relies on
 * a `struct HeapAlloc` that exposes the system allocator as an instance of `Alloc`
 * an integration of `Alloc` with `RawVec`
 * ~~an integration of `Alloc` with `Vec`~~

 TODO
 * [x] split `fn realloc_in_place` into `grow` and `shrink` variants
 * [x] add `# Unsafety` and `# Errors` sections to documentation for all relevant methods
 * [x] remove `Vec` integration with `Allocator`
 * [x] add `allocate_zeroed` impl to `HeapAllocator`
 * [x] remove typedefs e.g. `type Size = usize;`
 * [x] impl `trait Error` for all error types in PR
 * [x] make `Layout::from_size_align` public
 * [x] clarify docs of `fn padding_needed_for`.
 * [x] revise `Layout` constructors to ensure that [size+align combination is valid](https://github.com/rust-lang/rust/pull/42313#issuecomment-306845446)
 * [x] resolve mismatch re requirements of align on dealloc. See [comment](https://github.com/rust-lang/rust/pull/42313#issuecomment-306202489).

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