Const translation doesn't understand indexing into non-entire slices #3169
Description
Currently the consts translation pass can only handle indexing into a slice if it represents an entire vec, not a sub-vec thereof. Since we don't (presently) have any syntax for making a sub-slice as a const expr, this isn't a big deal. Yet. But it might become one in the future. If so, we need to do a fair bit more work in the const translator, since it appears LLVM doesn't want to feed us back the number-values of consts that depend in any way on the size of a datum. Such as, say, a size bound.
Awkward.