Extends our modelling to partially cover the behaviour of
`copy.replace`. In particular, we model this in two ways:
Firstly, we extend the existing Models-as-Data row for `copy` and
`deepcopy` to also cover `replace`. This means that we treat the result
of `replace` as containing all of the fields of the original object.
This is somewhat _more_ than we want, as strictly speaking the fields
that are overwritten should _not_ propagate flow through the `replace`
call, but currently we don't have a good way of modelling this blocking
of flow.
Secondly, we add a flow summary that adds flow from named arguments of
the `replace` call to the corresponding fields on the base object. This
ensures that we at least have the new flow arising from the `replace`
call.
Note that the flow summary adds this flow for _all_ named arguments of
_all_ `replace` calls throughout the codebase. However, since any
particular `replace` call will only populate a subset of these (the
subset consisting of exactly those named arguments that are in that
particular call), this does not cause any unwanted crosstalk between
different `replace` calls.§
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