Allow Solution to be variable length to permit regtest use.

[zancas]

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[LarryRuane]

Hi Za, I assume #505 is why this PR is needed; sorry I broke the testnet use case -- I never knew anyone used it with testnet. Let me know if I can be of any help with this PR. Taking a quick look now, it looks reasonable (changing Solution back to a byte slice instead of a byte array).