repr(Rust) page doesn't reflect the latest data layout optimizations

[Lokathor]

The page about repr(Rust) says that we don't have much in the way of data layout optimization "as of rust 1.0", but as of
PR 45225 we got a lot more going on in the optimization department.

[Gankra]

yep, would be nice to explain some of these things.

[JohnTitor]

The note has been removed by #43.