Make IoSlice and IoSliceMut methods unstably const

[thaliaarchi]

This enables constructing IoSlice and IoSliceMut in const contexts without users needing to break the platform abstraction by doing the cast themselves. This is useful for statics.

Since all current and plausible future platforms use simple slice-like structs, this should not impose a burden when adding a new platform with a different struct.

[rustbot]

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[thaliaarchi]

If this looks good, I'll open a tracking issue and link it.

[ibraheemdev]

r? libs-api

[thaliaarchi]

@BurntSushi Friendly ping. Would you mind taking a look at this?

[tgross35]

This seems fine; if we wind up with an obscure platform for which the vector type can't be constructed at comptime for some reason, we would probably use the unsupported fallback instead.

Nominated.

[tgross35]

For after discussion and updates,
r? libs

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.