isvalid() chr2ind() ind2chr() for SubString of DirectIndexString

On a microbenchmark these sped up isvalid(SubString{DirectIndexString},i) by about 40%
and produced a 4-5x speed improvement in chr2ind(SubString{DirectIndexString},i)
and ind2chr(SubString{DirectIndexString},i)

Benchmark details + code_typed() comparisons are available at: http://nbviewer.ipython.org/gist/catawbasam/99043bb74519378428fd