inverted fields in `hex::Error::InvalidLength`

[RCasatta]

I am testing https://github.com/ElementsProject/rust-elements/pull/188 in a downstream lib

https://github.com/ElementsProject/rust-elements/blob/613204f38eeb8f4fcc767042197fd763965f057a/src/hex.rs#L33

I think the expected, got comment is wrong and the values used in the Display implementation are inverted.

I say so by looking at the macro impl after in a downstream lib I have this test failing to parse "aaaa" as asset id with

  left: "bad hex string length 64 (expected 4)"
 right: "bad hex string length 4 (expected 64)"

(the test was working with add54c80ba1821a16b88af59de16009f28379853)

[apoelstra]

I think we can replace src/hex.rs entirely once #188 is merged. Or I can add a commit to 188 next week.