Improve and simplify fits Section indexing.

Add support for indexing fits Sections with slices with nontrivial (step != 1) slices, with integer arrays, and with boolean arrays.
Point being, I don't think it's necessary to build a big array of indices just to consume them.  I feel like there has to be a better way, and that I've done it myself before.  I'll look into this more when I have a chance.  Either way, overall I don't think this will hold up this PR forever--I still want to merge sooner rather than later :)