add RawValue for deferring/omitting serde-ing

[CobaltCause]

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Unfortunately this requires some unsafe, but fortunately it's only two lines and it's very easy to follow. There are some problems still though, such as not being able to deserialize to a &RawValue and Box<RawValue>'s deserialize impl doing a roundtrip through Value. RawValue could probably also be made available on alloc-only no-std platforms, but I'm leaving it at std-only for now just in case I'm wrong.

That aside, it does what it's supposed to do, so that's nice.

Also, it seems like this repository won't be getting any new activity but I thought I'd open this PR just for visibility if nothing else.