`Duration::from_secs_{f32, f64}` now use the results from the
non-panicking functions and unwrap it.

This also adds the error type used, `FromSecsError` and its `impl`s.

For reasoning, see https://github.com/rust-lang/rust/issues/63567#issuecomment-858640987

This commit performs two changes to stabilize Rust support for
WebAssembly simd intrinsics:

* The stdarch submodule is updated to pull in rust-lang/stdarch#1179.
* The `wasm_target_feature` feature gate requirement for the `simd128`
  feature has been removed, stabilizing the name `simd128`.

This should conclude the FCP started on #74372 and...

Closes #74372

Co-authored-by: NYuki Okushi <jtitor@2k36.org>

Add a map method to std::ops::range::Bound, patterned off of the method
of the same name on Option

This is to address issue 84297.

This is preparation for reverting 81238 for short-term resolution of issue 84297.

 ## User-facing changes

- Intra-doc links to primitives that currently go to rust-lang.org/nightly/std/primitive.x.html will start going to channel that rustdoc was built with. Nightly will continue going to /nightly; Beta will link to /beta; stable compilers will link to /1.52.1 (or whatever version they were built as).
- Cross-crate links from std to core currently go to /nightly unconditionally. They will start going to /1.52.0 on stable channels (but remain the same on nightly channels).
- Intra-crate links from std to std (or core to core) currently go to the same URL they are hosted at; they will continue to do so. Notably, this is different from everything else because it can preserve the distinction between /stable and /1.52.0 by using relative links.

Note that "links" includes both intra-doc links and rustdoc's own
automatically generated hyperlinks.

 ## Implementation changes

- Update the testsuite to allow linking to /beta and /1.52.1 in docs
- Use an html_root_url for the standard library that's dependent on the channel

  This avoids linking to nightly docs on stable.

- Update rustdoc to use channel-dependent links for primitives from an
  unknown crate

- Set DOC_RUST_LANG_ORG_CHANNEL from bootstrap to ensure it's in sync
- Include doc.rust-lang.org in the channel

This reverts commit 37a5b515.

`next_back` is a method of DoubleEndedIterator, not Iterator.