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/return_at_end.wat

@@ -0,0 +1,10 @@
+(module
+  (memory 1)
+  (func $main (param i32)
+      (if
+          (get_local 0)
+          (then (return))
+          (else (unreachable))
+       )
+  )
+)