inverted fields in hex::Error::InvalidLength

[RCasatta]

I am testing #188 in a downstream lib

rust-elements/src/hex.rs

Line 33 in 613204f

/// Tried to parse fixed-length hash from a string with the wrong type (expected, got).

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 add54c8)

[apoelstra]

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