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.

tests/wasm/exit125_wasi_snapshot0.wat

@@ -0,0 +1,8 @@
+(module
+  (import "wasi_unstable" "proc_exit"
+    (func $__wasi_proc_exit (param i32)))
+  (func $_start
+    (call $__wasi_proc_exit (i32.const 125))
+  )
+  (export "_start" (func $_start))
+)

tests/wasm/exit125_wasi_snapshot1.wat

@@ -0,0 +1,8 @@
+(module
+  (import "wasi_snapshot_preview1" "proc_exit"
+    (func $__wasi_proc_exit (param i32)))
+  (func $_start
+    (call $__wasi_proc_exit (i32.const 125))
+  )
+  (export "_start" (func $_start))
+)

tests/wasm/exit126_wasi_snapshot0.wat

@@ -0,0 +1,8 @@
+(module
+  (import "wasi_unstable" "proc_exit"
+    (func $__wasi_proc_exit (param i32)))
+  (func $_start
+    (call $__wasi_proc_exit (i32.const 126))
+  )
+  (export "_start" (func $_start))
+)

tests/wasm/exit126_wasi_snapshot1.wat

@@ -0,0 +1,8 @@
+(module
+  (import "wasi_snapshot_preview1" "proc_exit"
+    (func $__wasi_proc_exit (param i32)))
+  (func $_start
+    (call $__wasi_proc_exit (i32.const 126))
+  )
+  (export "_start" (func $_start))
+)

tests/wasm/exit2_wasi_snapshot0.wat

@@ -0,0 +1,8 @@
+(module
+  (import "wasi_unstable" "proc_exit"
+    (func $__wasi_proc_exit (param i32)))
+  (func $_start
+    (call $__wasi_proc_exit (i32.const 2))
+  )
+  (export "_start" (func $_start))
+)

tests/wasm/exit2_wasi_snapshot1.wat

@@ -0,0 +1,8 @@
+(module
+  (import "wasi_snapshot_preview1" "proc_exit"
+    (func $__wasi_proc_exit (param i32)))
+  (func $_start
+    (call $__wasi_proc_exit (i32.const 2))
+  )
+  (export "_start" (func $_start))
+)