Port to new TranslationUnit.executor

[alexfikl]

This caches the executor in _loopy_transform_cache. Is that ok?

[inducer]

Thanks!

I was working on also using the executor stuff in the lazy codepath, which required a bit of a detour. Otherwise, I already have analogous changes lined up (with the addition of also using the executor business on freeze).

Caching in _loopy_transform_cache is just fine: That's specific to an array context, which is itself specific to a CL context, so there's no risk of holding an uninvited reference to a CL context.

Still, I'll close this in favor of my version of this. Thanks again for making this, and sorry about the duplicate work.

[inducer]

Superseded by #241.