Change proc_exit to unwind the stack rather than exiting the host process. (#1646)

* Remove Cranelift's OutOfBounds trap, which is no longer used.

* Change proc_exit to unwind instead of exit the host process.

This implements the semantics in https://github.com/WebAssembly/WASI/pull/235.

Fixes #783.
Fixes #993.

* Fix exit-status tests on Windows.

* Revert the wiggle changes and re-introduce the wasi-common implementations.

* Move `wasi_proc_exit` into the wasmtime-wasi crate.

* Revert the spec_testsuite change.

* Remove the old proc_exit implementations.

* Make `TrapReason` an implementation detail.

* Allow exit status 2 on Windows too.

* Fix a documentation link.

* Really fix a documentation link.

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

@@ -342,13 +342,6 @@ pub(crate) struct FdEventData<'a> {
     pub(crate) userdata: wasi::__wasi_userdata_t,
 }
 
-pub(crate) fn proc_exit(_wasi_ctx: &WasiCtx, _memory: &mut [u8], rval: wasi::__wasi_exitcode_t) {
-    trace!("proc_exit(rval={:?})", rval);
-    // TODO: Rather than call std::process::exit here, we should trigger a
-    // stack unwind similar to a trap.
-    std::process::exit(rval as i32);
-}
-
 pub(crate) fn proc_raise(
     _wasi_ctx: &WasiCtx,
     _memory: &mut [u8],