Allow INDEX? of NONE, returning NONE

This makes INDEX? "NONE-transparent", in line with the already
none-transparent EMPTY? and LENGTH?:

    >> index? none
    == none

This implements CureCode wish #1611.
Remove redundant code

The A_INDEXQ and A_LENGHTQ cases would already fall through to A_TAKE
containing precisely the same `if` (making the `if` redundant in case
the condition is true) and then falls through further to the `trap_it`
block (making the `goto` redundant in case the `if` condition is false).
Move NONE! to end of INDEX? & LENGTH? typespecs
Merge pull request #67 from earl/allow-index-of-none

Allow INDEX? of NONE!