Add Base.length methods for ViewAxis and ComponentAxis types

[dingraha]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

This PR just adds Base.length methods for the ViewAxis and ComponentIndex types, which I find necessary in downstream packages.

[visr]

This would be good to reduce the breakage from #249. I'm not sure how internal it was, but getting a ComponentIndex instead of an Int and a ViewAxis instead of a UnitRange seems pretty breaking.

Perhaps it would make sense to add these as well:

• Cannot convert an object of type ComponentArrays.ComponentIndex{Int64, ComponentArrays.NullAxis} to an object of type Int64
• MethodError: no method matching iterate(::ComponentArrays.ViewAxis{1:0, nothing, ComponentArrays.Shaped1DAxis{(0,)}})

[dingraha]

@visr Sorry for the trouble. I've added a couple of methods that may fix your issue to this PR. Would you be able to try them out?

[visr]

@dingraha thanks a lot, that fixes all the issues we noticed with v0.15.23!