topics: standardize `strings` topic
topics: standardize `strings` topic

There were quite a few topics that were string related
and not listed in the [topics list](https://github.com/exercism/problem-specifications/blob/master/TOPICS.txt).

* Replace all of them with `strings` which is in the list.
* Add `parsing` if the topic was `string_parsing`.

petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust,petertseng/exercism-rust,exercism/xrust