The field are always re-ordered to minimize padding, regardless of the
alignment of the discriminant

This commit changes some usage of mem::forget into mem::ManuallyDrop
in some Vec, VecDeque, BTreeMap and Box methods.

Before the commit, the generated IR for some of the methods was
longer, and even after optimization, some unwinding artifacts were
still present.

rustc_target: Some cleanup to `no_default_libraries`

Do not disable field reordering on enums with big discriminant

The field are always re-ordered to minimize padding, regardless of the
alignment of the discriminant

(spinoff from rust-lang#70477)

Match options directly in the Fuse implementation

Rather than using `as_ref()`, `as_mut()`, and `?`, we can use `match` directly to save a lot of generated code. This was mentioned as a possibility in rust-lang#70366 (comment), and I found that it had a very large impact on rust-lang#70332 using `Fuse` within `Chain`. Let's evaluate this change on its own first.

…Mark-Simulacrum,RalfJung

use ManuallyDrop instead of forget inside collections

This PR changes some usage of `mem::forget` into `mem::ManuallyDrop` in some `Vec`, `VecDeque`, `BTreeMap` and `Box` methods.

Before the commit, the generated IR for some of the methods was longer, and even after optimization, some unwinding artifacts were still present.