[BUG] ambiguity checks and fixes

[lkdvos]

Aqua still reports quite a number of ambiguity warnings that should be fixed at some point. Most of these involve getindex in some form

[mtfishman]

I think #13 will help with that, since we can convert named indices to unnamed indices in Base.to_indices and get rid of a lot Base.getindex overloads.

[lkdvos]

I am guessing the separate IndexStyle is also something that has to be checked, because the API of that is badly documented within Base, and there might just be missing overloads/disambiguations for that as well.