implement multiple inheritance

[jrevels]

brought to you by: yesterday's MCO -> JFK plane ride

see the added MultV1 test in tour.jl for a simple example

Still needs:

• proper deprecation of Legolas.parent
• updates to enable existing test suite to pass (+ fixes for any bugs caught by existing test suite)
• new tests to cover the multiple inheritance functionality and various edge cases that probably arise
• more comprehensive tour examples
• updates to existing documentation/docstrings
• a new section in the manual providing modeling guidance about when to use multiple inheritance vs. "foreign key-ish" relationships (e.g. using junction records)
• an implementation of a has_matching_ancestor utility function (since this is now more complicated for downstream users to hand-roll)

Would be very useful if folks could try this PR out in situ in downstream use cases and report back any encountered challenges / bugs / MWEs

[jrevels]

#106 will presumably land before this, in which case this should be bumped again