Move back to only one WASI submodule (#1434)

* Move back to only one WASI submodule

This commit fixes the issue where we have two WASI submodules for build
reasons in this repository. The fix was to place the submodule in the
`wasi-common` crate, and then anyone using the `wig` crate has to be
sure to define a `WASI_ROOT` env var in a build script to be able to
parse witx files.

With all that in place `wasi-common` becomes the source of truth for the
witx files we're parsing, and crates like `wasmtime-wasi` use
build-scripts shenanigans to read the same witx files. This should
hopefully get us so we're compatible with publishing and still only have
one submodule!

* rustfmt

crates/wasi-common/src/old/snapshot_0/host.rs

@@ -8,7 +8,7 @@ use crate::old::snapshot_0::wasi::*;
 use std::{convert::TryInto, io, mem, slice};
 use wig::witx_host_types;
 
-witx_host_types!("old/snapshot_0" "wasi_unstable");
+witx_host_types!("phases/old/snapshot_0/witx/wasi_unstable.witx");
 
 pub(crate) unsafe fn ciovec_to_host(ciovec: &__wasi_ciovec_t) -> io::IoSlice {
     let slice = slice::from_raw_parts(ciovec.buf as *const u8, ciovec.buf_len);

crates/wasi-common/src/old/snapshot_0/mod.rs

@@ -9,7 +9,7 @@ pub mod wasi;
 pub mod wasi32;
 
 pub mod hostcalls {
-    wig::define_hostcalls!("old/snapshot_0" "wasi_unstable");
+    wig::define_hostcalls!("phases/old/snapshot_0/witx/wasi_unstable.witx");
 }
 
 pub use ctx::{WasiCtx, WasiCtxBuilder};

crates/wasi-common/src/old/snapshot_0/wasi.rs

@@ -8,7 +8,7 @@
 
 use wig::witx_wasi_types;
 
-witx_wasi_types!("old/snapshot_0" "wasi_unstable");
+witx_wasi_types!("phases/old/snapshot_0/witx/wasi_unstable.witx");
 
 pub type WasiResult<T> = Result<T, WasiError>;
 

crates/wasi-common/src/old/snapshot_0/wasi32.rs

@@ -12,4 +12,4 @@ use wig::witx_wasi32_types;
 pub type uintptr_t = u32;
 pub type size_t = u32;
 
-witx_wasi32_types!("old/snapshot_0" "wasi_unstable");
+witx_wasi32_types!("phases/old/snapshot_0/witx/wasi_unstable.witx");