Remove Select trait

I realized that our `select` implementation predated `Simd` being generic over element type, and we don't really need the `Select` trait at all. The function signature is much simpler now (generic over element type, rather than over the entire vector). This did require changing mask select to be a different function, but I think that's fine considering they're not necessarily vectors.

This ensures that the output does not depend on environment variables
set in the shell.

The FromIterator impl made the code much harder to understand. The types
don't make sense until you realize there's a custom FromIterator impl.

Remove `in_band_lifetimes` from `rustc_incremental`

rust-lang#91867

Remove P: Unpin bound on impl Stream for Pin

Similar to rust-lang#81363.

Sync portable-simd to fix libcore build for AVX-512 enabled targets

Fixes rust-lang#91484 (comment)
cc `@workingjubilee`

…, r=Mark-Simulacrum

Set `RUST_BACKTRACE=0` when running location-detail tests

This ensures that the output does not depend on environment variables
set in the shell.

Add a space and 2 grave accents

I only noticed this because I have this implementation copy pasted in some places in my code and I really can't wait for this to be stabilized...

rustdoc: Write doc-comments directly instead of using FromIterator

The FromIterator impl made the code much harder to understand. The types
don't make sense until you realize there's a custom FromIterator impl.

This is the first commit from rust-lang#91305; since `@camelid` wrote it originally I don't feel bad unilaterally approving it.

r? `@ghost`
`@bors` r+

Note that this will conflict with rust-lang#92078.