make headSize and tailSize back into ArrowOpts fields

As attributes, there are situations where it is actually impossible (or
at least very difficult) to get things the way you want.  E.g. imagine
creating a whole bunch of arrows via connectOutside and applyAll, but
you want them to have all different head sizes.  I am not sure how to
accomplish this via attributes at the moment.

It is also simply annoying to have some arrow attributes set via
ArrowOpts and some as normal attributes.  I ran into this when updating
the website gallery, it was a confusing error and would have been
annoying to fix.

In the long term, I would love to have all arrow attributes be normal
attributes... but that may have to wait until we have constraint solving
and can declare arrows as separate standalone diagrams (to which
attributes can be applied).

Note this does mean deleting headSizeG, headSizeO, etc. It would be
ideal to be able to write e.g. `... & headSizeG .~ 3 & ...`
rather than `... & headSize (Global 3) & ...`, however, such a
`headSizeG` would not be a valid lens.
Merge pull request #177 from diagrams/ht-size

make headSize and tailSize back into ArrowOpts fields