Add support for emitting code with a single return at the end. (#153)

This also enables testing of the wasmtests tests.

This also updates for wabt updating to the official "wat" filename
extension, as opposed to "wast".

cranelift/wasmtests/globals.wat

@@ -0,0 +1,8 @@
+(module
+  (global $x (mut i32) (i32.const 4))
+  (memory 1)
+  (func $main (local i32)
+    (i32.store (i32.const 0) (get_global $x))
+  )
+  (start $main)
+)