`wit-parser::decoding::decode` should error when given a core module

[rylev]

wit-parser::decoding::decode requires a WebAssembly component (either as a normal component or as a WIT-package-encoded-as-binary). However, if you pass in a core module, everything "works" (i.e., no error is returned), but you end up with an empty resolve. I imagine the right thing to do here would be to error specifying that a component must be passed.

Let me know if this is the case, and I can submit a fix.

[alexcrichton]

One use case of passing a core module is that there's these *.o files that come out of wit-bindgen with world information and I wanted to support wasm-tools component wit on these files. I don't believe that's tested in this repository though and I'm not sure if it still works at all. That leads me to:

• If that doesn't work, then I think it's reasonable to just return an error saying that core modules aren't supported.
• If that does work, then a core module with zero custom sections that look like component-type* should return an error

[ChristofPetig]

I have used wasm-tools component wit on core modules containing the section in the past and it worked fine. Given the wasit2 idea (core modules with WIT interfaces, see WASI issue #595) I would recommend to go for the second choice.