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/call.wat

@@ -0,0 +1,10 @@
+(module
+  (func $main (local i32)
+    (set_local 0 (i32.const 0))
+    (drop (call $inc))
+  )
+  (func $inc (result i32)
+    (i32.const 1)
+  )
+  (start $main)
+)