qualify generated reference to Predef.identity. fixes #211

[dsilvasc]

The generated object Implicits contains this line:

implicit val showString = build[String](identity)

When the generated code lives in a package foo.bar.baz and another package foo.identity exists in the classpath, the generated code fails to compile with:

Implicits.scala:21: package identity is not a value
    implicit val showString = build[String](identity)

Changing identity to Predef.identity would avoid the issue.

I tried also _root_.Predef.identity but that fails to compile with object Predef is not a member of package <root>.

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[blast-hardcheese]

Great, thanks!