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/memory.wat

@@ -0,0 +1,11 @@
+(module
+  (memory 1)
+  (func $main (local i32)
+    (i32.store (i32.const 0) (i32.const 0x0))
+    (if (i32.load (i32.const 0))
+        (then (i32.store (i32.const 0) (i32.const 0xa)))
+        (else (i32.store (i32.const 0) (i32.const 0xb))))
+  )
+  (start $main)
+  (data (i32.const 0) "0000")
+)