Fix param of Type::from_xdr_base64

[leighmcculloch]

What

Change Type::from_xdr_base64 fn to accept any type that as-refs to [u8] instead of accepting a String.

Why

There's three reasons.

The first is that by accepting a String we're requiring the caller to provide an owned value to the function. The function doesn't need an owned value, so the request for an owned value is unnecessary.

The second is that by accepting a String we're requiring that the caller to provide a value that is stored in memory allocated by the global allocator (the heap). Again this is unnecessary.

The third is that the base64 may not be stored in a string, it might have been communicated over a network or some other medium where we've got it in bytes, and converting it to a string before passing it to the decoder is unnecessary work that has fallible cases to handle because Rust strs are required to be UTF-8.

There is no cost introduced with this change. The same approach on other types is generated in this way and it appears to be a mistake we didn't do it the same way here.