Document eta-expansion workaround for Empty : Set

I thought I had pushed this before (see
https://github.com/HoTT/HoTT/pull/371#issuecomment-41201465).  But I
guess no.
Make lift and lower transparent

The reason to make them opaque was purely for typeclasses.  They will
need to be transparent when we move to trunk-polyproj, to get an
alternate version of Funext_downward_closed to work correctly.
Give a better error message on a tactic
Merge pull request #386 from JasonGross/transparent-lift-lower

Make lift and lower transparent
Add an instance about [idtoiso] and [components_of]
Add some typeclass resolution hints for apply being stupid

[apply] doesn't know that, e.g., the identity functor applied to x is
x, nor vice-versa.  And because it infers arguments in the "wrong"
order, it errors before it has a chance to pick up that information.
So we need to help it along a little.
Add some lemmas and tactics for removing isos

This is the main tactic used to prove that lax comma categories are
precategories.  Unfortunately, it's sometimes rather slow.  It could
probably benefit from reflective automation.

This closes #383