wasi-sockets: Add network-error-code

[badeend]

Implements WebAssembly/wasi-sockets#105

This function downcasts a stream error to a network-related error.
Similar to the existing filesystem-error-code and http-error-code functions.

Unit test is missing because I couldn't figure out a reliable way to fail a TCP stream mid-connection. Suggestions are welcome.

[alexcrichton]

Thanks for this! Before I think too hard about a unit test though this PR is the unfortunate victim of "first proposal to add WITs with @unstable". As implemented this provides unrestricted access to the @unstable version of the API which is not the intention of the annotation. The theory is that @unstable would translate to some sort of -S flag such as -Snetwork-error-code or something like that. That would be there during development and then when @unstable is turned to stable then -Snetwork-error-code becomes a noop effectively.

I say that this is an "unfortunate victim" because this infrastructure hasn't been built yet. Ideally what needs to happen I think is that the calls to add_to_linker all need to be parameterized by feature flags where the default is the empty set. So in a sense we need a set of flags for all WASI proposals which are passed during the original call to add_to_linker and then that set is built up on the CLI.

From a technical perspective though this PR otherwise looks good to me. Would you be up for helping out with the infrastructure for plumbing flags through?

[badeend]

I'll give it a try

[badeend]

Do you have any preference on how to thread through the feature names down into the generated code? I have three suggestions so far:

• Add component_model_unstable_features setting to wasmtime::Config
• Add unstable_features setting to wasmtime::component::Linker
• Add features parameter to generated add_to_linker_* methods and all layers below and above it.

[alexcrichton]

I'd recommend the add_to_linker_* route and it would automatically be used when features is specified in the bindgen! macro (it's a WASI-specific or bindings-specific thing, less about engine state)