PERF: Optimize loadtxt usecols.
PERF: Optimize loadtxt usecols.

7-10% speedup in usecols benchmarks; it appears that even in the
single-usecol case, avoiding the iteration over `usecols` more than
compensates the cost of the extra function call to usecols_getter.

charris/numpy,mhvk/numpy,mattip/numpy,anntzer/numpy,jakirkham/numpy,numpy/numpy,numpy/numpy,rgommers/numpy,mhvk/numpy,simongibbons/numpy,mhvk/numpy,seberg/numpy,rgommers/numpy,mattip/numpy,anntzer/numpy,rgommers/numpy,seberg/numpy,simongibbons/numpy,pdebuyl/numpy,endolith/numpy,simongibbons/numpy,jakirkham/numpy,rgommers/numpy,pdebuyl/numpy,jakirkham/numpy,mhvk/numpy,charris/numpy,numpy/numpy,simongibbons/numpy,simongibbons/numpy,pdebuyl/numpy,endolith/numpy,pdebuyl/numpy,endolith/numpy,mattip/numpy,mattip/numpy,anntzer/numpy,endolith/numpy,mhvk/numpy,seberg/numpy,jakirkham/numpy,anntzer/numpy,seberg/numpy,jakirkham/numpy,charris/numpy,charris/numpy,numpy/numpy