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 `Ident(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 seems to force the enclosing module and blow up.

Review by @paulp

 - have I lost something by using `Ident(classSym)` here.
 - in general, when would one choose `REF(_)` over `Ident(_)`?