This makes the code less noisy and is a preparation for changing it to
`const`-based literal. Because of the preparation, places that used
variables to store the hex string were changed to constants.
There are still some instances of `Vec::from_hex` left - where they
won't be changeable to `const` and where `hex!` is unavailable
(integration tests). These may be dealt with later.
See also #1189
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