Rollup merge of #143921 - oli-obk:const-index, r=fee1-dead

Constify `Index` traits

tracking issue: rust-lang/rust#143775

the `SliceIndex` trait cannot be implemented by users as it is sealed. While it would be useful for the `get` method on slices, it seems weird to have a feature gate for that that isn't also gating index syntax at the same time, so I put them under the same feature gate.

r? ```````@fee1-dead```````

Author: samueltardieu