f98192af55
This enables indexing in `!foreach` and permutation with `list[permlist]`.
Enhancements in syntax:
- `list<int>` is applicable as a slice element.
- `list[int,]` is evaluated as not `ElemType` but `list<ElemType>`
with a single element.
Part of D145872
FIXME: I didn't apply new semantics to BitSlice.