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

This commit polishes the new proc introduced by d9ed816b10.

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.

changelog.md

@@ -145,6 +145,9 @@
 
 - Export `asyncdispatch.PDispatcher.handles` so that an external library can register them.
 
+- Added `deques.toDeque`, which creates a deque from an openArray. The usage is
+  similar to procs such as `sets.toHashSet` and `tables.toTable`. Previously,
+  it was necessary to create an empty deque and add items manually.
 
 ## Language changes
 - The `=destroy` hook no longer has to reset its target, as the compiler now automatically inserts