Add a symbolOf function

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A very frequent use for a result of typeOf is obtaining an underlying type symbol. Another thing that comes up occasionally at stack overflow is a request to add facilities for reification of symbols.

This naturally suggests that our reflection API would benefit from a method called symbolOf that can take a term or type argument and return an underlying symbol.

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Imported From: https://issues.scala-lang.org/browse/SI-8194?orig=1
Reporter: @xeno-by