quasireducible are also known as lazyreducible.
There is a lot of work to be done.
We still need to revise blast, and add a normalizer for type class
instances. This commit worksaround that by eagerly unfolding
quasireducible.
After this commit, Lean "cuts" the search after the first instance is
computed. To obtain the previous behavior, we must use the new command
multiple_instances <class-name>
closes#370