SI-7033 Be symful when creating factory methods.

Implicit class factory methods were synthesizing the
reference to the class as `Ident(classDef.name)`, which
was unhygienic in case of `implicit class X[X]`.

To use symbols without causing a cycle, I switched from
`REF(symbol)` to `mkUnattributedRef(symbol)`. The former
calls into:

    at scala.reflect.internal.TreeGen.mkAttributedSelect(TreeGen.scala:184)
    at scala.reflect.internal.TreeGen.mkAttributedRef(TreeGen.scala:124)
    at scala.reflect.internal.TreeGen.mkAttributedRef(TreeGen.scala:130)
    at scala.tools.nsc.ast.TreeDSL$CODE$.REF(TreeDSL.scala:307)

which forces the info of enclosing module and forms a cycle.