Converts a org.json4s.ast.fast.JValue to a org.json4s.ast.safe.JValue.
Converts a org.json4s.ast.fast.JValue to a org.json4s.ast.safe.JValue. Note that when converting org.json4s.ast.fast.JString, this can throw runtime error if the underlying string representation is not a correct number
JNumber is internally represented as a string, to improve performance