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