Efficient get_dim_size for arbitrary dimensions

This implements an efficient method get_dim_size(i) to get an individual dimension, as discussed at https://github.com/ContinuumIO/libdynd/issues/78
I just meant that the ``if (i < get_ndim()) {`` test already guarantees that it's not built-in if ``i`` is a valid >= 0, so this if statement can be removed.