Provide a macro to find a particular struct in a struct chain

[kainino0x]

Related to #272 where we discuss the few cases where, depending on the outcome of that issue, we may return a chained struct to the application (rather than the application constructing one and passing it to the implementation).

Providing FindInChain utilities would be nice, though not blocking. FWIW, I think it can even be done type-safely using _Generic in C11 and templates in C++. A little programming puzzle for someday.

Calling could look something like:

// unsafe version with manual cast
const WGPUMyExtension* extension = (const WGPUMyExtension*) WGPU_FIND_IN_CHAIN(root, WGPUSType_MyExtension);

or

// safe version using _Generic or template to look up the WGPUSType
const WGPUMyExtension* extension;
WGPU_FIND_IN_CHAIN(root, extension);