write to inherent_impls during the visitor

The goal here is to avoid writing to the `inherent_impls` map from
within the general `Coherence` task, and instead write to it as we
visit. Writing to it from the Coherence task is actually an information
leak; it happened to be safe because Coherence read from
`DepNode::Krate`, but that was very coarse.

I removed the `Rc` here because, upon manual inspection, nobody clones
the data in this table, and it meant that we can accumulate the data in
place. That said, the pattern that is used for the inherent impls map
is *generally* an anti-pattern (that is, holding the borrow lock for the
duration of using the contents), so it'd probably be better to
clone (and I doubt that would be expensive -- how many inherent impls
does a typical type have?).

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