This PR adds the new attributes `[grind =>]` and `[grind <=]` for
controlling pattern selection and minimizing the number of places where
we have to use verbose `grind_pattern` command. It also fixes a bug in
the new pattern selection procedure, and improves the automatic pattern
selection for local lemmas.
The tests `grind_constProp.lean` and `no_grind_constProp.lean` are the
same use case with and without `grind`.
This PR adds a few builtin case-splits for `grind`. They are similar to
builtin `simp` theorems. They reduce the noise in the tactics produced
by `grind?`.
