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/utils.rs

@@ -1,4 +1,4 @@
-use proc_macro2::{Literal, TokenStream, TokenTree};
+use proc_macro2::{Ident, Literal, TokenStream, TokenTree};
 
 /// Given the input tokens to a macro invocation, return the path to the
 /// witx file to process.
@@ -53,3 +53,13 @@ fn parse_string_literal(literal: Literal) -> String {
 
     trimmed
 }
+
+pub fn param_name(param: &witx::InterfaceFuncParam) -> Ident {
+    quote::format_ident!(
+        "{}",
+        match param.name.as_str() {
+            "in" | "type" => format!("r#{}", param.name.as_str()),
+            s => s.to_string(),
+        }
+    )
+}