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/src/instantiate.rs

@@ -5,6 +5,7 @@ use cranelift_wasm::DefinedFuncIndex;
 use std::fs::File;
 use std::rc::Rc;
 use target_lexicon::HOST;
+use wasi_common::hostcalls;
 use wasi_common::wasi;
 use wasi_common::{WasiCtx, WasiCtxBuilder};
 use wasmtime_environ::{translate_signature, Export, Module};