Rust: Allow implicit reads from reference content in taint reach.

[geoffw0]

Allow implicit reads from reference content in the taint reach calculation:

• the metric was designed without content in mind. As written nodes reached with content on them do not count as reached in the metric, which leads to undercounting.
• however allowing any content produces a huge cartesian product in allowImplicitRead.
• as a compromise, allow implicit read for reference content, which I hypothesize may be responsible for a big part of the undercounting (DCA to confirm).

[geoffw0]

DCA shows no increase in taint reach. I'd better take a closer look at what's going on...

[geoffw0]

Allowing implicit reads of TuplePositionContent has a bigger impact, but it's not very principled to pick just that one out. As mentioned above, allowing implicit reads of any content breaks performance (cartesian product). Think I'll just close this and make do with the metric as it is.