SPELLING-OF as native function

[hostilefork]

This adds the SPELLING-OF native from the rebol-proposals repository.
It works on ANY-STRING! or ANY-WORD!, and extracts the "pre-formed"
string data from them:

>> spelling-of <abc>
"abc"

>> spelling-of quote def:
"def"

>> spelling-of "ghi"
"ghi"

Very easy to write as a native, somewhat harder to engineer from outside
the system (especially as Rebol2 would to-string on a SET-WORD! as
the spelling, while R3-Alpha had the delimiter...)