The abstraction offered by this function is that of an infinitely-long list
consisting of repeating copies of the input list, but previously we
restricted it only to positive indices.

Now we'll also treat the indices before zero as a negative extension of
the infinite list, for consistency.

Technically the list is not actually infinite but rather constrained by
the range of an int64 in Go. Ideally we'd relax that too and allow any
whole number that's in the full range of cty.Number, but there's no clear
benefit to that beyond a theoretical idea of correctness and so we'll
save that one for a later change if we discover a real use-case for it.

When descending into an object inside a Transformer, the marks from the
outer object should not be applied to the attribute values. This follows
the pattern used in other collection types, and ensures marks are stable
across multiple transformations.