Use string_view's instead of char pointers where applicable

[Eyal Rozenberg]

It would be nicer/cleaner for us to hold string views - essentially pointers + lengths - rather than raw char *'s or const char *'s.

[Eyal Rozenberg]

Not that useful, considering how NVRTC and OpenCL take null-terminated strings, meaning that, with string_view's, we'd need to construct null-terminated strings to pass.