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/hostcalls_impl/misc.rs

@@ -326,3 +326,18 @@ pub(crate) struct FdEventData<'a> {
     pub(crate) r#type: wasi::__wasi_eventtype_t,
     pub(crate) userdata: wasi::__wasi_userdata_t,
 }
+
+pub(crate) fn proc_exit(_wasi_ctx: &WasiCtx, _memory: &mut [u8], rval: wasi::__wasi_exitcode_t) {
+    trace!("proc_exit(rval={:?})", rval);
+    // TODO: Rather than call std::process::exit here, we should trigger a
+    // stack unwind similar to a trap.
+    std::process::exit(rval as i32);
+}
+
+pub(crate) fn proc_raise(
+    _wasi_ctx: &WasiCtx,
+    _memory: &mut [u8],
+    _sig: wasi::__wasi_signal_t,
+) -> Result<()> {
+    unimplemented!("proc_raise")
+}