Add struct chaining docs

[kainino0x]

Also add compile tests for the things I wrote in the documentation sample code.

Issue: none, #264, should have been listed in #25, related to #463

[lokokung]

nit. To clarify this a bit more (at least w.r.t the fuzzer case), it's not just that the same sType can't be in the same chain, but that the same sType should never appear in a chain or member chain on a root struct. Not sure how to word it clearly here...

[kainino0x]

(discussed offline)

The first requirement (sType is valid for the type of the chain root) should be the one that validates that. The only gap is if some implementation adds an extension that can actually recurse, which we probably should not allow them to do, so I'll note that.

We should also guarantee that if the implementation doesn't allow the sType in that context, it will never try to downcast the struct. It's the same as if the sType were totally bogus.

In Dawn's wire, Loko came up with a way to guarantee that invalid chains can be serialized and deserialized without hitting serialization errors: if the client doesn't allow an sType in context, convert it to something like WGPUSType_DawnInvalidChainedStruct which indicates struct WGPUDawnInvalidChainedStruct { WGPUChainedStruct chain; WGPUSType sTypeForErrorMessage; }. As long as the client and server are in sync the server can always handle this without a deserialization error, and forward it into dawn-native which will produce a validation error. If the client is out of sync or is malicious (e.g. a fuzzer) then if the server sees an sType that's not valid in context it immediately gives up and fails deserialization.

[kainino0x]

The only gap is if some implementation adds an extension that can actually recurse, which we probably should not allow them to do, so I'll note that.

Actually I started trying to write this down and realized there could be legitimate use cases that pass variable-sized data as a linked list (rather than an array or array of pointers), regardless of whether they're using struct chaining. So I'm going to make this a "should", not a "must".