Change all the strided dims to use the same arrmeta

This changes the arrmeta layout of cfixed_dim and fixed_dim,
making the match strided_dim. This means the dim size and
strided are represented redundantly in these cases, but allows
code dealing with strided dimensions to treat them all
the same.
Add strided_ndim property to ndt::type properties, other strided

Created size_stride_t pair struct, for the arrmeta structure
common to the strided dimensions. Added a multidimensional
tp.get_as_strided(...) variant to complement the single dimension
one.
Merge pull request #84 from mwiebe/strided_arrmeta

Unify arrmeta of strided/cfixed/fixed dim types