minimize: Implement `mir::ProjectionElem::ConstantIndex`

[Indeximal]

In some slice or string tests I ran into this rvalue. I have also partly implemented this in https://github.com/minirust/minirust/pull/240/commits/3e41862eb2b2e24acc1cd880d8231be20338a0fc. However, the current function architecture does not allow to properly check UB when the min_length field is not met, therefore this has been reverted.