After this commit, new interactice tactic classes can be added without
writing C++ code (see example: tests/lean/run/my_tac_class.lean).
The tactic_evaluator was simplified, and all the complexity has been
moved to tactic_notation, and lean code.
We can now inspect the intermediate states produced by the rewrite
tactic.
The function (@scope_trace _ line col thunk) can be used to position trace
messages produced by thunk. If line/col are not provided (i.e., we
just write (scope_trace thunk)), then line/col are filled with the
position of this term by the elaborator.
We can visualize the intermediate tactic states inside nested blocks
such as (try { ... })
The new infrastructure can be used to implement custom tactic_state
pretty printers.
In the standard library, we should use explicit universe variables for
universe polymorphic definitions.
Users that want to declare universe polymorphic definitions but do not
want to provide universe level parameters should use
Type _
or
Type*
