functions on xtensor_fixed enhancements

[wolfv]

We have the cool xtensor fixed -- now it's time to do some flashy things with it :)

The cool thing for most of these will be that we can allocate the required memory statically.

Some things we could do:

• [x] compute shape of xreducers statically
• [ ] compute shape of xviews statically
• [x] forward shape of xaccumulators statically
• [x] eval() of xfunction with xfixed leave returns xfixed

[wolfv]

We should also propagate the xtensor_fixed shape in xfunction, so that xt::eval(xf1 + xf2) results in a xtensorf of the correct shape.

This might involve metaprogramming to determine broadcastability at compile time, as well as the resulting shape.

[wolfv]

gonna close this with the merge of #922 and open another issue for the view things