Inline String::into_raw_parts and reuse from_utf16 optimization

[fereidani]

1. I noticed that comment // This isn't done via collect::<Result<_, _>>() for performance reasons. explicitly states that this approach is wrong, but from_utf16le and from_utf16be are not using the same optimization.

2. I believe into_raw_parts is missing the #[inline] as any other similar functions in the standard library are marked either with inline or inline(always).

This is also related to #48994; whenever that fix lands, optimizing from_utf16_units will results in these 3 functions getting optimized.

[rustbot]

r? @clarfonthey

rustbot has assigned @clarfonthey.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: libs
• libs expanded to 12 candidates
• Random selection from 6 candidates