OFArray_adjacent: Optimize fast enumeration

This restores the previous behaviour for OFArray_adjacent. It was lost
when OFArray's fast enumeration was changed to be better optimized for
huge arrays that aren't adjacent in memory. I forgot that
OFArray_adjacent just used the implementation from OFArray, as OFArray
had the ideal implementation for OFArray_adjacent, but now that OFArray
is optimized for huge, non-adjacent arrays, this wasn't the case
anymore.