Update wit-bindgen and bump to 0.13.0

[alexcrichton]

This commit updates wit-bindgen to 0.21.0 which brings in some minor updates to generated code in addition to a new wit-bindgen-rt crate dependency to use as well. This avoids the need to depend on the wit-bindgen crate at all through some updated options to the CLI bindings generator.

Additionally this bumps the crate to 0.13.0 to release these updated change in addition to the refactoring for the export-related macros previously.

[sunfishcode]

This commit updates wit-bindgen to 0.21.0 which brings in some minor updates to generated code in addition to a new wit-bindgen-rt crate dependency to use as well. This avoids the need to depend on the wit-bindgen crate at all through some updated options to the CLI bindings generator.

What is the advantage of not depending on wit-bindgen? We depend on bitflags either way, and wit-bindgen itself appears to have very little in it other than re-exports of wit-bindgen-rt. Is it the code in the pre_wit_bindgen_0_20_0 module?

[alexcrichton]

Oh no real advantage per-se, but I figure it's at least a bit nicer in terms of "fewer deps" and better aligns with what cargo component is doing. AFAIK there's no technical benefit for doing so.

[sunfishcode]

Why is wit-bindgen-rt a separate crate? Would it make sense to just fold it into wit-bindgen?

[alexcrichton]

That was motivated by bytecodealliance/cargo-component#241 where I believe the issue was that cargo add was conflicting with the default dependency injected by cargo component and it was easier to manage if cargo component didn't have to deal with the features of a wit-bindgen dependency at all.