Add std_detect::detect::features() API that returns iterator of target-features and reject unstable features in feature-detection macros

[gnzlbg]

This PR implements an unstable features() function in std::detect that returns an iterator over the target features, returning a tuple of the target-feature name, and whether it is enabled or disabled in the current host.

This can be used to implement a solution to rust-lang/rust#54688 using our own run-time feature detection system.

The refactor required to implement this also allows us to close#667

[lu-zero]

Do you plan to rebase this? I could use something like this to map the env var content to the features.

[gnzlbg]

Yep, I'll merge this today.

[lu-zero]

It looks fine and I'd use it

[lu-zero]

It rebases cleanly but if I try to build it I get:

 Checking core_arch v0.1.5 (/Users/lu_zero/Sources/rust/stdarch/crates/core_arch)
error[E0658]: non-builtin inner attributes are unstable
--> crates/core_arch/src/simd.rs:3:1
|
3 | #![rustfmt::skip]
| ^^^^^^^^^^^^^^^^^
|
= note: for more information, see https://github.com/rust-lang/rust/issues/54726
= help: add `#![feature(custom_inner_attributes)]` to the crate attributes to enable

[gnzlbg]

Damn I forgot about this.

[lu-zero]

Actually I had a problem with my local repo and origin/master was pointing to the wrong place, so a rebase isn't immediate :)

(if you have time to rebase and push would be great)

[lu-zero]

If it survives I guess it should be good to go pending some shuffling/amending.

[gnzlbg]

Gonna update stdarch upstream and do a crater run. There might be code in the wild accidentally relying on some of the unstable features.