deques.nim: Refactor the `toDeque` functionality (#15166)

This commit polishes the new proc introduced by d9ed816b10a6.

Changes:
- Rename to `toDeque` for more consistency with well-known procs like
  `toHashSet` and `toTable`.
- Rename the `openArray` parameter. The name `arr` was potentially less
  clear given that the proc can be used with a seq (or string).
- Add a `since` annotation.
- Reword the doc comment, and clarify that ordering is preserved.
- Add runnableExamples.
- Add "see also" cross linking between `initDeque` and `toDeque`.
- Remove duplicate `nextPowerOfTwo`. The `initImpl` template already
  includes it.
- Implement the proc using the `items` iterator, rather than indexing.
  This matches the implementation of `sets.toHashSet` and
  `tables.toTable`.
- Add a test within `when isMainModule`.
- Add a changelog entry.

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