Remove unsafe from generated helpers and const-generic functions (#244)

Follow-up to #236

Didn't even need to extend `kernel!` to support const generics in the
end.

The only remaining `unsafe` is now in `load/store_interleaved` which can
be eliminated on x86 and WASM but not NEON. That's addressed separately
in #245

Fixes #124

Author: Shnatsel