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/wig/src/lib.rs

@@ -1,5 +1,6 @@
 extern crate proc_macro;
 
+mod hostcalls;
 mod raw_types;
 mod utils;
 mod wasi;
@@ -36,3 +37,8 @@ pub fn witx_wasi32_types(args: TokenStream) -> TokenStream {
 pub fn define_add_wrappers_to_module(args: TokenStream) -> TokenStream {
     wasi::add_wrappers_to_module(args.into()).into()
 }
+
+#[proc_macro]
+pub fn define_hostcalls(args: TokenStream) -> TokenStream {
+    hostcalls::define(args.into()).into()
+}