Add new topics to exercises related to data structures

[alirezaghey]

Issue #667 proposed normalizing the topics across exercises which was addressed in PR #711.

data_structures as a topic was removed from seven exercises, since it doesn't have a corresponding topic.

At least some of these seven exercises, and may be even other exercises, seem good candidates for a tag like data_structures or similar, since they cover this topic.

The seven exercises are as follows:

• matrix
• linked list
• list ops
• spiral matrix
• custom set
• circular buffer
• simple linked list

In my humble opinion, data structures, like algorithms, is a topic of interest to many students, and having them combined under one meaningful topic is helpful.

Before adding a new topic to this exercises addressing the subject of data structures, an issue must be opened at problem-specifications to add the missing topic.

[SleeplessByte]

@ErikSchierboom @F3PiX @kytrinyx @iHiD I think these all match one of our "exercise categories". This would probably be a possible moment to add this as a topic?

[ErikSchierboom]

Sounds like a good plan. I've checked the existing topics list and it indeed doesn't include this topic

[kytrinyx]

I'll add it to the problem specifications document now.

[alirezaghey]

I like to go through all the JS exercises and add the topic to those where it seems reasonable. Is that OK?

[SleeplessByte]

@alirezaghey yes please. I think the seven you listed are prime candidates.