Remove superfluous UbChecks from `SliceIndex` methods
The current implementation calls the unsafe ones from the safe ones, but that means they end up emitting UbChecks that are impossible to hit, since we just checked those things.
This PR adds some new module-local helpers for the code shared between them, so the safe methods can be small enough to inline by avoiding those extra checks, while the unsafe methods still help catch length mistakes.
r? `@saethlin`
cd0c944b07