transmute in assign_nested

[Licenser]

Describe the problem you are trying to solve

The assign nested function takes care of assigning values in the form of:

let some.nested.value[2..6][2]["key"]

The reasoning to use transmute here is listed in the code as follows:

        /* NOTE
         * This function is icky we got to do some trickery here.
         * Since it's dangerous and icky it deserves some explanation
         * What we do here is we borrow the target we want to set
         * as immutable and mem::transmute it to mutable where needed.
         *
         * We do this since there is no way to tell rust that it's safe
         * to borrow immutable out of something that's mutable even if
         * we clone data out.
         *
         * This is safe because:
         *
         * We only borrow Cow<'event, str> out of the host. So the
         * reference points to either the event or script and we
         * never mutate strings only ever replace them.
         * So even if the map the Cow originally came from we won't
         * lose the referenced data. (Famous last words)
         */

Describe the solution you'd like

No sensible re-write comes to mind here, but it's good to have this noted. Perhaps there are approaches we haven't thought of. As with other low-level optimizations, this will be resolved when tremor-script becomes a VM or compiled language.

Notes
Followup to #1015