This PR eliminates uses of `intros x y z` (with arguments) and updates
the `intros` docstring to suggest that `intro x y z` should be used
instead. The `intros` tactic is historical, and can be traced all the
way back to Lean 2, when `intro` could only introduce a single
hypothesis. Since 2020, the `intro` tactic has superceded it. The
`intros` tactic (without arguments) is currently still useful.
Then `isClassExpensive?` was being invoked too often. In some
benchmarks the performance hit was substantial. For example,
in the new test `state8.lean`. The runtime on my machine went from 2s
to 0.76s.