Co-authored-by: Josh Triplett <josh@joshtriplett.org>

For our kernel targets, we should not set OS, as the kernel runs bare
metal without a circular dependency on std.

This also prepares us for unifying with
rust-lang#89062. This patch requires
libhermit-rs to change a `cfg`s from `target_os = "hermit"` to `target_os
= "none"`.

I tested this patch locally.

…r=joshtriplett

Add #[must_use] to thread::Builder

I copied the wording of the [`fmt::Debug` builders](https://doc.rust-lang.org/src/core/fmt/builders.rs.html#444).

Affects:

```rust
std/src/thread/mod.rs:289:5   std::thread::Builder   fn new() -> Builder;
std/src/thread/mod.rs:318:5   std::thread::Builder   fn name(mut self, name: String) -> Builder;
std/src/thread/mod.rs:341:5   std::thread::Builder   fn stack_size(mut self, size: usize) -> Builder;
```

Parent issue: rust-lang#89692

r? `@joshtriplett`

…plett

Add #[must_use] to remaining alloc functions

I've run out of compelling reasons to group functions together across crates so I'm just going to go module-by-module. This is everything remaining from the `alloc` crate.

I ignored these because they might be used to purposefully leak memory... or other allocator shenanigans? I dunno. I'll add them if y'all tell me to.

```rust
alloc::alloc          unsafe fn alloc(layout: Layout) -> *mut u8;
alloc::alloc          unsafe fn alloc_zeroed(layout: Layout) -> *mut u8;
alloc::sync::Arc<T>   fn into_raw(this: Self) -> *const T;
```

I don't know why clippy ignored these. I added them myself:

```rust
alloc::collections::btree_map::BTreeMap<K, V>   fn range<T: ?Sized, R>(&self, range: R) -> Range<'_, K, V>;
alloc::collections::btree_set::BTreeSet<T>      fn range<K: ?Sized, R>(&self, range: R) -> Range<'_, T>;
```

I added these non-mutating `mut` functions:

```rust
alloc::collections::btree_map::BTreeMap<K, V>     fn range_mut<T: ?Sized, R>(&mut self, range: R) -> RangeMut<'_, K, V>;
alloc::collections::btree_map::BTreeMap<K, V>     fn iter_mut(&mut self) -> IterMut<'_, K, V>;
alloc::collections::btree_map::BTreeMap<K, V>     fn values_mut(&mut self) -> ValuesMut<'_, K, V>;
alloc::collections::linked_list::LinkedList<T>    fn iter_mut(&mut self) -> IterMut<'_, T>;
alloc::collections::linked_list::LinkedList<T>    fn cursor_front_mut(&mut self) -> CursorMut<'_, T>;
alloc::collections::linked_list::LinkedList<T>    fn cursor_back_mut(&mut self) -> CursorMut<'_, T>;
alloc::collections::linked_list::LinkedList<T>    fn front_mut(&mut self) -> Option<&mut T>;
alloc::collections::linked_list::LinkedList<T>    fn back_mut(&mut self) -> Option<&mut T>;
alloc::collections::linked_list::CursorMut<'a, T> fn current(&mut self) -> Option<&mut T>;
alloc::collections::linked_list::CursorMut<'a, T> fn peek_next(&mut self) -> Option<&mut T>;
alloc::collections::linked_list::CursorMut<'a, T> fn peek_prev(&mut self) -> Option<&mut T>;
alloc::collections::linked_list::CursorMut<'a, T> fn front_mut(&mut self) -> Option<&mut T>;
alloc::collections::linked_list::CursorMut<'a, T> fn back_mut(&mut self) -> Option<&mut T>;
```

I moved a few existing `#[must_use]`s from functions onto the iterator types they return: `IntoIterSorted`, `IntoKeys`, `IntoValues`.

Parent issue: rust-lang#89692

r? `@joshtriplett`

…plett

hermit: Implement Condvar::wait_timeout

This implements `Condvar::wait_timeout` for the `hermit` target.

See
* hermit-os#2
* hermit-os#5

CC: `@stlankes`

…shtriplett

hermitkernel-target: Set OS to "none"

For our kernel targets, we should not set OS, as the kernel runs bare
metal without a circular dependency on std.

This also prepares us for unifying with
rust-lang#89062. This patch requires
libhermit-rs to change a `cfg`s from `target_os = "hermit"` to `target_os
= "none"`.

I tested this patch locally.

CC: `@stlankes`