feat: support raw_slice returns from scripts and contracts (#743)

This PR closes #703 by adding support for `raw untyped slice` in
scripts.
It adds the possibility of returning a vector from a script, as
basically an array of `u64`.

Co-authored-by: Ahmed Sagdati <37515857+segfault-magnet@users.noreply.github.com>
Co-authored-by: Anton Trunov <anton.a.trunov@gmail.com>
Co-authored-by: Halil Beglerović <git@hal3e.io>