Add support for Embedded Swift:

• Audit our platform-specific code and add branches for Embedded Swift based on what we can assume is available in a standards-compliant libc.
• Get embedded hardware (Raspberry Pi or some such)
• Set up CI jobs

Above and beyond other porting efforts:

• Figure out how to run tests on an embedded system (cross-platform build-and-run using SwiftPM)
• Figure out what features are possible with Embedded Swift and which will be unavailable