Update to wit-bindgen 0.20.0

[alexcrichton]

This commit updates the bindings generator to 0.20.0 and refactors the existing support for exporting macros to use the new features of 0.20.0 as well. The macros crate feature is now no longer required (yay!) and all the traits to implement are already present in the crate now.

[alexcrichton]

cc @lann to double-check I'm not missing anything from #77

[lann]

Nice!

My only comment is about naming; from an external perspective its a little odd that the macro names don't include the package name. If it was e.g. http_proxy_export it would line up nicely between wasi:http/proxy and wasi::http_proxy_export.

I don't think its critically important for the wasi package itself but this crate probably does set a bit of a precedent for other similar crates.

[alexcrichton]

Good point! I've reverted back to the names you added prior, with one change to rename the incoming_handler module to proxy instead to reflect a bit how the world is wasi:http/proxy. Makes me think I should probably update wit-bindgen to perhaps do this by default.

[lann]

Makes sense to me. Should the run mod be renamed to command too?

[alexcrichton]

Oop yes, good point