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

Cargo.lock

@@ -1860,20 +1860,6 @@ version = "0.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "efd1f82c56340fdf16f2a953d7bda4f8fdffba13d93b00844c25572110b26079"
 
-[[package]]
-name = "trybuild"
-version = "1.0.19"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "987d6fdc45ddd7f3be5aa7386c8c8a844d1655c95b9ed948a9cd9cded8f2b79f"
-dependencies = [
- "glob",
- "lazy_static",
- "serde",
- "serde_json",
- "termcolor",
- "toml",
-]
-
 [[package]]
 name = "typemap"
 version = "0.3.3"
@@ -1992,23 +1978,12 @@ dependencies = [
  "log",
  "num",
  "thiserror",
- "wasi-common-cbindgen",
  "wig",
  "winapi",
  "winx",
  "yanix",
 ]
 
-[[package]]
-name = "wasi-common-cbindgen"
-version = "0.9.0"
-dependencies = [
- "proc-macro2 1.0.7",
- "quote 1.0.2",
- "syn 1.0.13",
- "trybuild",
-]
-
 [[package]]
 name = "wasm-webidl-bindings"
 version = "0.6.0"
@@ -2427,11 +2402,13 @@ dependencies = [
 
 [[package]]
 name = "witx"
-version = "0.6.0"
+version = "0.7.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "abc432537dbc9940e06816ebc3e1c4694fc835b90720615c32038769d7b2967d"
+checksum = "a6ee25990cb94f39b8d5637b8c7d9b9eaaed76795d95f45342a7ef3d2e574931"
 dependencies = [
  "clap",
+ "log",
+ "pretty_env_logger",
  "thiserror",
  "wast 3.0.4",
 ]