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/src/lib.rs

@@ -1,3 +1,5 @@
+use wasmtime::Trap;
+
 pub mod old;
 
 pub use wasi_common::{WasiCtx, WasiCtxBuilder};
@@ -12,3 +14,17 @@ pub fn is_wasi_module(name: &str) -> bool {
     // trick.
     name.starts_with("wasi")
 }
+
+/// Implement the WASI `proc_exit` function. This function is implemented here
+/// instead of in wasi-common so that we can use the runtime to perform an
+/// unwind rather than exiting the host process.
+fn wasi_proc_exit(status: i32) -> Result<(), Trap> {
+    // Check that the status is within WASI's range.
+    if status >= 0 && status < 126 {
+        Err(Trap::i32_exit(status))
+    } else {
+        Err(Trap::new(
+            "exit with invalid exit status outside of [0..126)",
+        ))
+    }
+}