Update our use of the term "layout"

As of [this PR](https://github.com/rust-lang/unsafe-code-guidelines/pull/457/files#diff-3d27114a58daccf7b7133bf5d8552c310e4399a5f4df29fe8eefcef7e4ec18ceR115-R117), the Rust reference defines the term "layout" as:

> The *layout* of a type defines its size and alignment as well as the offsets of its subobjects (e.g. fields of structs/unions/enums/... or elements of arrays, and the discriminant of enums).
>
> Note that layout does not capture everything that there is to say about how a type is represented on the machine; it notably does not include ABI or Niches.

Our existing documentation and comments use the term "layout" to refer only to a type's size and alignment. We should update our usage to be consistent with this definition.

Tasks:

- [ ] Find any location where we use the term "layout"
- [ ] Make sure its use is consistent with this definition. A few considerations:
- In doc comments, it may be appropriate (depending on context) to link to this definition
- In some contexts, it may be appropriate to call out that field offsets are included in the definition (many readers will not be familiar with the precise definition)
- We must not use "layout" in a context in which we _don't_ mean to include field offsets. In such cases, we should explicitly say "size and alignment" or similar.