make Uuid #[repr(transparent)]

[kinggoesgaming]

Based on #461 by @DianaNites

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The crate supports no_std, but the Uuid type isn't FFI safe/repr(transparent)? There doesn't seem to be any reason for it not to be. That attribute is stable since 1.28.0, which is below the crates MSRV.

[KodrAus]

Uuid isn't currently #[repr(transparent)] so that it doesn't leak its internal layout. Arrays are also a bit strange for FFI, so folks that need to use a #[repr(transparent)] Uuid in a FFI would need to pass it as &Uuid. This isn't necessarily a reason not to add a #[repr(transparent)] though.