[Rust] Extract Concepts from v2 exercise: parallel-letter-frequency

[lewisclement]

Extract the list of concepts required to solve the v2 Rust exercise: parallel-letter-frequency. For each concept, describe why it is required to solve the exercise. Keep in mind that an exercise can have multiple approaches.

The list of concepts should be stored in the following document: languages/rust/reference/exercise-concepts/parallel-letter-frequency.md. Use this link to create a PR to add this document.

It can be helpful to also include a sample solution in the document.

[lewisclement]

Closed per #253