Configure WIT feature gates at runtime & implement wasi-cli exit-with-code

[badeend]

For extra context, see: #9276

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This PR adds the ability to enable/disable WIT APIs gated by @unstable(feature = ...) at link-time.

Previously, unstable features could only be controlled at compile-time through the features option of wasmtime::component::bindgen!. That option has been removed. Bindings for unstable APIs are now generated unconditionally. Instead, the generated add_to_linker*** functions take an additional LinkOptions parameter if the world/interface has any unstable features. The availability of unstable features is now controlled through that.

The component bindgen generates a fresh LinkOptions type for each interface that needs it and a single LinkOptions type for the entire world, which is a union of all the options contained in that world. An example is included in the component-macro tests: unstable-features.wit produces:
https://github.com/badeend/wasmtime/compare/245dcab34c2fcae69e0f6c3f2daa15837ea6ff61..4fcfe6f866f36ed663f24199e1744bed3cf0bc2c#diff-ad635965474c257cbafce510463aaf52c1ac813f22168045ae22546d3b3ebfc6

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A happy coincidence is that WASI-0.2.1 already includes a tiny but practical first use case: exit-with-code.
I've implemented that function and is gated behind the -Scli-exit-with-code CLI flag.

@sunfishcode, if I interpret WebAssembly/wasi-cli#11 correctly, you were OK with removing the Windows-specific restriction, right? See: badeend@2f48c77

[alexcrichton]

This looks great to me, thanks for this!

In retrospect though given the large number of changes related to add_to_linker what do you think about:

• Interface/world-level add_to_linker retains the LinkOptions parameter
• Top-level add_to_linker loses the LinkOptions parameter, and there's a new top-level add_to_linker_with_options function?

Or perhaps this would only affect the helpers we have in wasmtime-wasi and wasmtime-wasi-http? Basically it looks like there should be a convenience function for "default options" rather than requiring all callers to call it.

[badeend]

• Interface/world-level add_to_linker retains the LinkOptions parameter
• Top-level add_to_linker loses the LinkOptions parameter, and there's a new top-level add_to_linker_with_options function?

Don't know what top-level function you're referring to. I think the Interface/world-level add_to_linker functions are the only ones generated by the bindgen?

I have just updated the wasmtime-wasi's functions back to their old signature and added an extra set of _with_options variants. I didn't touch the add_to_linker functions generated by bindgen.

[alexcrichton]

Ah sorry for my rambling, this is exactly what I was thinking 👍

[badeend]

Try again please

[badeend]

Sigh.

Try again, again?