Introducing the Embedded Swift Platform Abstraction Layer

Hi all, tl;dr we're making some changes to the way Embedded Swift interacts with the platform and it will require small changes to existing Embedded Swift programs to continue to link properly. In our discussions with various people using Embedded Swift, one of the issues we keep hearing about is that it's hard to tell exactly what functions you need to provide to get Embedded Swift to run on top of your platform. There is a documented set of external dependencies, but it's a somewhat ad hoc s...