Auto-generate the hostcalls module of wasi-common (#846)

* Auto-generate shims for old `wasi_unstable` module

This commit is effectively just doing what #707 already did, but
applying it to the `snapshot_0` module as well. The end result is the
same, where we cut down on all the boilerplate in `snapshot_0` and bring
it in line with the main `wasi_snapshot_preview1` implementation. The
goal here is to make it easier to change the two in tandem since they're
both doing the same thing.

* Migrate `wasi_common::hostcalls` to a macro

This commit migrates the `hostcalls` module to being auto-generated by a
macro rather than duplicating a handwritten signature for each wasi
syscall.

* Auto-generate snapshot_0's `hostcalls` module

Similar to the previous commit, but for `snapshot_0`

* Delete the `wasi-common-cbindgen` crate

This is no longer needed with the hostcalls macro now, we can easily
fold the definition of the cbindgen macro into the same crate.

* Rustfmt

* Fix windows build errors

* Rustfmt

* Remove now no-longer-necessary code

* rustfmt

crates/wasi-common/src/lib.rs

@@ -24,20 +24,21 @@
 mod ctx;
 mod error;
 mod fdentry;
-mod helpers;
-mod hostcalls_impl;
-mod sandboxed_tty_writer;
-mod sys;
-#[macro_use]
-mod macros;
 pub mod fs;
+mod helpers;
 mod host;
-pub mod hostcalls;
+mod hostcalls_impl;
 mod memory;
 pub mod old;
+mod sandboxed_tty_writer;
+mod sys;
 pub mod wasi;
 pub mod wasi32;
 
+pub mod hostcalls {
+    wig::define_hostcalls!("snapshot" "wasi_snapshot_preview1");
+}
+
 pub use ctx::{WasiCtx, WasiCtxBuilder};
 pub use sys::preopen_dir;