Missed layout optimization

[jrose]

Some previous discussion here, including notes from Swift:

Pre-RFC: Allow array stride != size

(Posted for: Separate size and stride for types · Issue #1397 · rust-lang/rfcs · GitHub) [Pre-RFC] Allow stride != size Summary Rust should allow for values to be placed at the next aligned position after the previous value, ignoring the tail padding of that previous field. This requires changing the meaning of "size", so that a value's size in memory (for the purpose of reference semantics and layout) is not definitionally the same as the distance between consecutive values of that type (its…