Rust: serde json encode [u8; N] as hex string

[leighmcculloch]

What

Change the serde json encoding of [u8; N] as a hex string in the Rust generated code.

Why

It is less common than encoding binary as base64, but the vast majority of our fixed type u8's in the Stellar XDR represent small values that we often share in hex format, so this puts the encoding of XDR closer to the native form we would typically discuss the values.

If this presents significant challenges then I think simply encoding as base64, like #118, would also be okay.

Follow up to #112 #117