Use std::byte for JIT buffers/views/associated helpers

We currently use `uint8_t*` for JIT input/output buffers. The unpacked views will `reinterpret_cast` `uint8_t*`, which could be (although is not on our supported platforms) UB. `std::byte` would be safer. This touches not just the JIT interface and views, but also a lot of associated helpers, e.g. for packing/unpacking, testing, etc.