Restrict qualify pointers in generated C code

[SamirDroubi]

Exo currently enforces a non-aliasing constraint on all data passed to procedures. We should restrict qualify all pointers passed into procedures and pointers defined in window structs.

Some care needs to be considered when doing this since restrict is only a keyword in C (since C99), but not in C++. Since headers can be included from either, we need to make sure to defined a portable restrict macro (similar to here: https://github.com/nemequ/hedley/blob/8fb0604a8095f6c907378cc3f0391520ae843f6f/hedley.h#L1500C20-L1500C28).

This also addresses #343