Updating the Embedded Swift vision

Hello Swift Community, The Language Steering Group has approved an update to the Embedded Swift vision. The specific changes can be seen in this pull request. The original Embedded Swift vision took a fairly hard stance that certain features (such as metatypes and existentials) and aspects of the generated code (such as type metadata) would never be present. As Embedded Swift has evolved, the philosophy has shifted: instead of outright prohibiting these features entirely, they are now availab...