As suggested, this reworks `Options` to subclass `T::Struct`, which
simplifies the implementation and makes it easier to maintain.
One noteworthy difference in switching to `T::Struct` is that
`#serialize` omits `nil` values but I don't _think_ this should be a
problem for us. In terms of changes, I modified `#merge` to remove a
now-unnecessary `compact` call and updated related tests.
Co-authored-by: Douglas Eichelberger <697964+dduugg@users.noreply.github.com>
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