Config : support providing full hex-encoded seeds via PROPTEST_RNG_SEED

[matthew-russo]

fixes #556

[moodmosaic]

@matthew-russo, thank you for working on this (via #556) 🙏 ❤️

Can we avoid requiring the hex- prefix for replay? The process is already
a bit complex. If the replay file is there, just use it. If not, the failure
message should mention how to replay with hex-. Also, there's
PROPTEST_RNG_SEED to consider. Adding another format rule adds friction.

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Better: auto-detect the seed format.
Try to parse the seed as a hex string of the right length.
If that works, use it as a hex seed.
Otherwise, parse it as a u64.

This lets users copy-paste seeds from failure output as-is.
Also works cleanly with PROPTEST_RNG_SEED.