Faster compilation of inductive implicits

[milessabin]

• Initial implementation — Faster compilation of inductive implicits. scala#5649
• Integration with byname implicits — in progress
• Document algorithm
• Improve compile time performance where heuristic inapplicable

From the initial PR,

This PR provides much faster (35 x in the provided benchmark case) compilation of the kind of inductive implicit resolution that is found in shapeless and its uses for type class derivation in libraries such as Circe, Doobie and Scodec.

[milessabin]

It turned out that almost all of the compile time superlinearity was accounted for by exploring solutions using polymorphic implicits which would never be applicable. In addition, unnecessary logging side effects added a huge constant factor. The former is fixed by early pruning of inapplicable polymorphic implicits and the latter by eliding the logging where possible.