…rect signedness

_span could possibly be confused with the Span type in rustc

Use a time representation with 1900-01-01-00:00:00 at timezone -1440 min as
anchor. This is the earliest time supported in UEFI.

Signed-off-by: Ayush Singh <ayush@beagleboard.org>

Add tests to ensure that extream system times are still representable.

Signed-off-by: Ayush Singh <ayush@beagleboard.org>

…medValueStr`

Modify `AttributeTemplate` to support list of alternatives for list and name value attribute styles.

Suggestions now provide more correct suggested code:

```
error[E0805]: malformed `used` attribute input
  --> $DIR/used_with_multi_args.rs:3:1
   |
LL | #[used(compiler, linker)]
   | ^^^^^^------------------^
   |       |
   |       expected a single argument here
   |
help: try changing it to one of the following valid forms of the attribute
   |
LL - #[used(compiler, linker)]
LL + #[used(compiler)]
   |
LL - #[used(compiler, linker)]
LL + #[used(linker)]
   |
LL - #[used(compiler, linker)]
LL + #[used]
   |
```

instead of the prior "masking" of the lack of this feature by suggesting pipe-separated lists:

```
error[E0805]: malformed `used` attribute input
  --> $DIR/used_with_multi_args.rs:3:1
   |
LL | #[used(compiler, linker)]
   | ^^^^^^------------------^
   |       |
   |       expected a single argument here
   |
help: try changing it to one of the following valid forms of the attribute
   |
LL - #[used(compiler, linker)]
LL + #[used(compiler|linker)]
   |
LL - #[used(compiler, linker)]
LL + #[used]
   |
```

This commit amends the documentation of `Vec::as_mut_ptr` and
`Vec::into_raw_parts` to make it explicit that such calls may be paired
with calls to `dealloc` with a suitable layout. This guarantee was
effectively already provided by the docs of `Vec::from_raw_parts`
mentioning `alloc`.

Additionally, we copy-paste and adjust the “Memory layout” section from
the documentation of `std::boxed` to `std::vec`. This explains the allocator
guarantees in more detail.

Reduce indentation and avoid needless checks (checking the target OS and
vendor is unnecessary).

This is more in-line with what Apple's tooling expects, and allows us to
better support custom compiler drivers (such as certain Homebrew and
Nixpkgs compilers) that prefer their own `-isysroot` flag.

Effectively, we now invoke the compiler driver as-if it was invoked as
`xcrun -sdk $sdk_name $tool`.

To allow using zig-cc or similar as the compiler driver.

The exact reasoning why we do not always pass the SDK root when linking
on macOS eludes me, but I suspect it's because we want to support
compiler drivers which do not support the `-isysroot` option.

Since we now pass the SDK root via the environment variable SDKROOT,
compiler drivers that don't support it can just ignore it.

Similarly, since we only warn when xcrun fails, users that expect their
compiler driver to provide the SDK location can do so now.

`rustc` is going to change the desugaring of `assert!` to be

```rust
match condition {
    true => {}
    _ => panic!(),
}
```
which will make the edge-case of `condition` being `impl Not<Output = bool>`
while not being `bool` itself no longer a straightforward suggestion,
but `!!condition` will coerce the expression to be `bool`, so it can be
machine applicable.

This is a technically a breaking change for what can be parsed in
`#[cfg(false)]`.