Allow index of none

This pull request primarily makes INDEX? "NONE-transparent", in line with the already none-transparent EMPTY? and LENGTH?:

    >> index? none
    == none

This implements [CureCode wish #1611](http://issue.cc/r3/1611).

Seeing as this was already once marked as "built" in CC before (but then later found to be not actually implemented), I assume this behaviour was actually intended to be implemented.

Aside from the INDEX? feature itself, the second commit removes some redundant code encountered while implementing the feature.