Implemengt NoPanicSlice in terms of non-panicing slicing in when it is available in Rust's standard library

[briansmith]

See rust-lang/rfcs#1325.

no_panic::NoPanicSlice::subslice, and maybe other things, should be rewritten when the new Rust functionality is available.

[briansmith]

It's now rust-lang/rfcs#1679.

[briansmith]

Now rust-lang/rust#35729

[sfackler]

Implementation: rust-lang/rust#36340

[briansmith]

It's not clear from reading #36340 whether it is possible to use the new feature on Stable rust without the SliceIndex trait being stable.

[briansmith]

This issue is moot now. First, the code was split out into the untrusted crate, and then we did do this in untrusted.