Update path separators to be available in const context Tracking issue: rust-lang/rust#153106 This makes platform-dependent secondary path separators available in const context (ie. at compile time). The platform definitions have also been consolidated behind a common macro to prevent transcription errors, whereas previously they were defined 3-4 times per platform. ### Questions I've manually verified that this compiles against each platform. It seems like no unit tests should be required for this change; is that correct?