unused: reduce number of calls to types.Implements
unused: reduce number of calls to types.Implements

types.Implements is a relatively expensive operation, producing a lot of
garbage in lookupFieldOrMethod.

When I tested 2019.1 on github.com/cockroachdb/cockroach/pkg/...,
lookupFieldOrMethod allocated 23 GB of garbage.

Updates gh-394

dominikh/go-tools,dominikh/go-tools,dominikh/go-tools,dominikh/go-tools