- remove `tupleStoreStep` and `dictStoreStep` from `containerStep`
These are imprecise compared to the content being precise.
- add implicit reads to recover taint at sinks
- add implicit read steps for decoders
to supplement the `AdditionalTaintStep`
that now only covers when the full container is tainted.
This test demonstrates the current state of affairs: that `copy.replace`
essentially blocks all flow of taint through it, because it has not been
modelled yet.