Add optional `allow_single_version` Boolean arg to `actx.compile`

Benefits:

- Avoids unexpected repeated compilation
- Avoid dispatch cost to select which implementation to use

Drawbacks:

- Less flexible, of course :)

Naming of the flag is an open question.

cc @majosm @mtcam

Start at the actx.compile entry point and read the issue's 12-comment discussion, since the flag naming and intended API details remain open. Confirm the option's behavior and name with the project maintainers before identifying the relevant implementation and tests; done means the agreed optional argument avoids unnecessary recompilation and dispatch cost.

索引モデルが issue の本文から書いたものです。