Rename namespace and some vector traits

To avoid future collisions with names in C++ standards and to disambiguate/align better with C++ naming scheme the following rename changes are suggested:
- change (or add nested namespace) namespace opencl for OpenCL specific type traits.
- rename some OpenCL vector traits as follows:
-- `vector_n` -> `make_opencl_vector`
-- `is_vector_type` -> `is_opencl_vector_type`
-- `vector_size` -> `opencl_vector_size`

Alternatively, we could use `simd_vector` instead of `opencl_vector`.

No files, tests, or entry points are named in the issue, so first locate the OpenCL type-trait definitions and their uses. Resolve whether the namespace and trait names should use `opencl` or `simd_vector`, then update all affected references and verify the library still builds and its tests pass.

Written by the indexing model from the issue text.