Rename inner `api` module to `Api`

[jakebolewski]

Modules by convention always start with an uppercase in Julia, so this has always felt out of place. This would be a breaking change so maybe it is not worth it.

[vchuravy]

I would say that the breakage currently would be acceptable, especially since we could add

import Api
const api = Api

into the OpenCL Module to minimize breakage.