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/Cargo.toml

@@ -17,10 +17,7 @@ proc-macro = true
 quote = "1.0.2"
 proc-macro2 = "1.0.6"
 heck = "0.3.1"
-# We include the WASI repo primarily for the witx files, but it's also useful
-# to use the witx parser it contains, rather than the witx crate from
-# crates.io, so that it always matches the version of the witx files.
-witx = "0.6.0"
+witx = "0.7.0"
 
 [badges]
 maintenance = { status = "actively-developed" }