The new command provides a uniform way to set declaration attributes.
It replaces the commands: class, instance, coercion, multiple_instances,
reducible, irreducible
See new tests (for examples)
tests/lean/run/coe10.lean
tests/lean/run/coe11.lean
tests/lean/run/coe9.lean
Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>