restrict type qualifier

In 22cbb7966fa16dae2a1dde14c3a7d9966661516c the `restrict` type qualifier was added to some variables, but Visual Studio doesn't like it. Microsoft uses [`__restrict`](https://learn.microsoft.com/en-us/cpp/cpp/extension-restrict) instead.

I guess we should make a `RESTRICT` macro to use in its place. I'd provide a pull request, but not sure where such a thing should live.

Start by reviewing commit 22cbb7966fa16dae2a1dde14c3a7d9966661516c to locate the variables using restrict, then check how compiler-specific definitions are organized. Confirm the replacement accommodates Visual Studio's __restrict and preserves the existing use of restrict elsewhere.

Written by the indexing model from the issue text.