Do our best to limit visiting paths to at most once, in preparation for
allowing multiple paths to be specified in a search-and-replace
operation. If we didn't do this, multiple path cases like this would be
handled incorrectly:

```
$ cat a.txt
foo

$ # We want to replace `foo` with `foobar`...
ruplacer foo foobar a.txt a.txt
<snip>

$ # ...but we got `foobarbar` instead:
$ cat a.txt
foobarbar
```

It's important to note what the trade-off for deduplication is. For each
path, we now call `std::fs::canonicalize`, and put the canonicalized
path in a set built using the the [`patricia_tree` crate] for somewhat
efficiently storage.

Interestingly, the above is also a potential issue with following
symlinks, which has never been allowed before. Perhaps this change could
make it sufficiently safe to add an option for following symlinks?

[`patricia_tree` crate]: https://docs.rs/patricia_tree/0.3.1/