Give wasmtime::component::Linker ability to stub out implementations

[rylev]

Feature

Several times I've simply wanted the linker to stub out all implementations for a component that it has not yet seen.

Essentially I've wanted to do roughly the following:

let mut linker = wasmtime::component::Linker::<MyData>::new(&engine);
wasmtime_wasi::add_to_linker_sync(&mut linker)?;
linker.stub_imports(&component)?;

With the above configuration, any imports configured by wasmtime_wasi would work correctly, but any other imports would simply trap when invoked.

Benefit

This would provide users a simple way to get the linker to be happy and turn instantiation errors into runtime errors. This can be useful in cases where you want to instantiate a component and call one of its exports, and you are fine with calls to certain imports to simply trap if they occur.

Implementation

The implementation should be fairly straight forward I believe through the use of Component::env_component to do the necessary reflection to know when to call LinkerInstance::func_new with a closure that calls panic!("stubbed function called").

Future possibilities

In the future, we could expose an additional API that would allow the user to supply a closure that is invoked on each import invocation so that the user can customize the behavior of the stub.

[alexcrichton]

Sounds reasonable to me! I think this'd be the equivalent of wasmtime::Linker::define_unknown_imports_as_traps for wasmtime::component::Linker.