It looks like JSONEncoder can't round-trip floating-point values on i386 only. Likely an x87 excess-precision bug, but I don't have time to investigate right now. Tracked by https://bugs.swift.org/browse/SR-9759
* Make SIMD types codable. We're considering this a bugfix.
This is a very tiny ABI change, in that user-defined SIMD types compiled with an earlier version of 5.0 will be missing the necessary conformance to Codable. Discussed with Ben, and we're OK with this because we don't think there are such types yet, and it can be fixed with a recompile.
* Add basic tests