Consider using Xargo and -Zalways-encode-mir

[eholk]

See the discussion on #58 for more context. @eddyb suggests that using xargo with -Zalways-encode-mir would let us avoid making our own tinycore and instead concentrate on using the actual Rust libcore. Since we choose what to translate, we can start from roots in the test cases we're compiling to avoid reaching code mir2wasm doesn't understand how to handle yet.