works around a seemingly spurious warning

[error] /Users/xeno_by/Projects/shapeless/examples/src/main/scala/shapeless/examples/staging.scala:111: inferred existential type mirror.universe.Expr[String] forSome { val mirror: reflect.runtime.universe.Mirror }, which cannot be expressed by wildcards,  should be enabled
[error] by making the implicit value scala.language.existentials visible.
[error] This can be achieved by adding the import clause 'import scala.language.existentials'
[error] or by setting the compiler option -language:existentials.
[error] See the Scala docs for value scala.language.existentials for a discussion
[error] why the feature should be explicitly enabled.
[error]     val consumeExpr = mkExpr[String](currentMirror)(consumeTree)
[error]                                                    ^

mkExpr's signature is:

  def mkExpr[T : TypeTag](mirror: Mirror)(tree : Tree) : mirror.universe.Expr[T] =

So I think the warning about its use is spurious:

  mkExpr[String](currentMirror)(consumeTree)