Add Engine::detect_precompiled_file()

[acfoltzer]

This is handy for users of deserialize_file() to determine which version to call.

[jameysharp]

I'm handing review of this off to Alex as the author of the API you're extending.

I had hoped that the from_trusted_file entry-point or similar would suffice to avoid embedders needing to detect for themselves whether they were holding on to a precompiled module. However, it sounds like both of you have reasons why that wasn't enough. I don't want to think about it that hard so I'll leave you to it.

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[alexcrichton]

Seems reasonable to me, thanks!

I had hoped that the from_trusted_file entry-point or similar would suffice to avoid embedders needing to detect for themselves whether they were holding on to a precompiled module

Ah yeah I tried to use this when I originally refactored the CLI to take components, but the main reason I couldn't use it was that the CLI needed to dispatch to either Component::deserialize or Module::deserialize depending on what the compiled bytes were. I initially hoped to avoid ModuleOrComponent in the wasmtime crate's API but I think from_trusted_file would work there.

Overall at least from the CLI I felt that our API for loading modules/components is still kinda clunky. I couldn't really figure out a better replacement though so I gave up and added detect_precompiled. I'm definitely all-ears though for other suggestions!

[acfoltzer]

I'm still getting acquainted with the component side of the crate API, but it seems like a ModuleOrComponent type is going to be necessary on the client side for pretty much any transitional project. If the expectation is that most applications will eventually move to the component model, I don't think it's worth codifying that sort of type in the crate API. But if you're feeling the pain while maintaining the CLI as well, it might be worth thinking more about some shared interfaces.